The cameras had finished grinding for the day and extras in a war movie being filmed in southern France began to assemble for dinner in a makeshift canteen. Their separate paths formed a tribute to what Charles de Gaulle called the External France.

The extras all came from the same village of the Midi. Most were friends. They had been assigned roles as officers or enlisted men on the basis of height and looks.

But as they moved across the dining room, the villagers playing French Army officers all grouped together at the same table. Across the room, the "enlisted" extras automatically sat down together.

It is a story Frenchmen tell about themselves, in their own blend of veneration and mistrust of the foibles that set them apart from other nationalities. It is a bittersweet story dedicated to France's thousand-year history of centralization, rigid class structures and excessive concern with authority.

The story comes out of the intense self-examination being conducted by the French as they approach one of the most crucial election campaigns in their recent history. Beyond the campaign, modern France is approaching a moment of definition as well as of choice.

That choice is framed by three decades of transformation that has passed unnoticed to many outside France. From a nation that proudly based its economy and its society on quality agriculture and small businesses, France has exploited Western Europe's most consistent rate of industrialization to grow into the world's fourth largest trading nation behind the United States, West Germany and Japan.

France today builds more cars than does West Germany. French consumer spending and per capita gross national product hover only slightly below the levels of the three other big manufacturing nations. French factory workers put in longer work weeks than do other laborers in the Common Market, and last year they lost half as much time through strikes as American workers did proportionately. Income Taxes Lost

THESE ARE statistic the government is proud to trumpet. There are others, from a different France, that are less pleasing to official ears.

It is a France in which 60 per cent of the income taxes due the government are lost through a system of tax evasion and avoidance that is more or less tolerated by the government, according to economist Pierre Eury. Personal income taxes bring in less than 15 per cent of the revenues available to the government, which depends on a brutally regressive value-added sales tax system that hits workers and the poor hardest.

It is a France with a wide gap between rich and poor, where the richest 10 per cent of the population absorbs nearly a third of the after-tax income of the nation while the poorest 10 per cent gets one-fiftieth of the national income, according to a recent study of countries in the Organization for Economic Cooperation and Development.

In the broadest sense, France is conducting a national debate on these two sets of statistics. The country is seeking to determined if the political structures that have come down through its long history, and more particularly from Charles de Gaulle, are workable after the recent sweeping industrial changes which have created a strong middle class enmeshed in consumer and social patterns that are more akin to other Western countries today than those of France three decades ago.

That middle class has yet to find the kind of centrist political expression that has emerged in other parts of Europe where deeply entrenched class and political structures have responded to change.

The conflict of politicians and parties erupting in France has envenomed the larger debate and helped obscure its more precise formulation: Is there a social democracy struggling to get out of the traditionally polarized French political system? Is there, in fact, room, or necessity, in France for the kind of managed capitalism that prevails in varying degrees in West Germany, Scandinavia and Britain?

The aloof aristocrat who inherited de Gaulle's position but not his gradeur, President Valery Giscard d'Estaing, feel there is. He argues that the rapid change in France has created a "floating" majority of essentially middleclass voters who want mild reforms in the system and whose interests are not represented by the ideological extremes that have dominated French politics in the past.

Giscard is basing his electoral strategy for the March, 1978. National Assembly elections around the analysis that France has to be governed by "the center." France no longer can afford to be "cut in half" by the struggle between parties dedicated to preserving or destroying the existing order of wealth and privilege, the president argues eloquently. Appeal to Emotions

MOST OF FRANCE'S other important political leaders remain unpersuaded, however.

"Voters in France do not correspond to social conditions," says Michel Rocard, the Socialist Party's top economic spokesman. "One-third of the manual laborers in France vote for the right wing. And one-third of the managerial class votes for the left today. Those who back the centrist notion are forgetting the terrific weight of history in France."

An associate of Gaullist leader Jacques Chirac puts the rightist view more succinctly. "Look, that centrist stuff is hogwash. The only way to get through to French voters is to appeal directly to their emotions, to get them by the genitals or by the tear glands."

Chirac makes no secret of his intention to dominate the March elections and eventually the country by continuing to stoke the ideological fires that Giscard wants to consign to France's past.

"On one point, the Communists and I agree," Chirac recently told a Gaullist party conference. "You do not go halfway down the road to communism."

Chirac has rebuilt the Gaullist party into a conventional political group held together by conservative ideology rather than the mystical appeal of de Gaulle. Chirac angrily resigned as Giscard's prime minister last year in the dispute over the president's hopes for a small opening to the left of the parliamentary coalition that has dominated the National Assembly since 1958.

Chirac thus has laid down a clear threat to Giscard's presidency from the right. Giscard has told friends that Chirac poses a greater immediate danger for the French republic than do the French Socialists or even the Communists, according to reliable reports.

The reported comment reflects not only the bitter antagonism, if not hatred, the two men feel for each other but also casts a long shadow over the choices available to Giscard once the March election returns are in.

The configuration of the next National Assembly will be key evidence in their personal battle for vindication, the result of a tooth-and-claw fight for power by the four main parliamentary groups - the Communists, Socialists, Gaullists and centrist supporters of Giscard. The Socialist Factor

THE MOST LIKELY results appear to promise Giscard a chance at fashioning a political center out of the fragmentation of the rightist and leftist blocs that may occur. But they also pose a risk that, without creative statesmanship. France is in for a new period of uncertainty and tension and a quick return to the polls for an even more highly polarized electoral battle.

The most decisive factor is almost certain to be the number of National Assembly seats captured by the Socialists, a politically and socially heterogeneous organization which has emerged from the murky political landscape the French call "the Swamp" to become the largest party in France today.

"The Swamp" occupies the space between the Communists and the Gaullists, who have been the durable poles of French politics World War II, when they fought each other while fighting the occupying German army. Each commands one-fifth to one-fourth of the popular vote, totals that are based in large part on unexamined class loyalties and on the lingering aura of the parties' Resistance records.

The Socialist Party was formed in 1971 from the bleached bones of the center-left and leftist parties that de Gaulle displaced when he came to power in 1958. The Socialists barely could claim 15 per cent of the vote then; today, national polls project a minimum 30 per cent share of the electorate for the Socialists and some estimates range up to 40 per cent.

Moreover, the Socialists have enlisted a disproportionately large share of France's brightest young civil servants, parliamentarians and academics under their banner.

"The barons of Gaullism were so afraid of being eliminated from power after the explosion of May, 1968, and after de Gaulle quit that they closed the organization in on itself," says 27-year-old Isabelle Mommessin, a disillusioned but still loyal Gaullist.

"Two things happened then," she adds. "The young managers and intellectuals who had no decision-making power in the present but who had hopes for the future went over to the left. And Chirac came as an outsider to take the Gaullists away from the barons."

The Socialists also gained credibility through the Common Program electoral pact they signed with the Communists in 1972. Without committing themselves to radical measures, they built up their image as a party that stood for social justice and the working class. And they clearly drew much of their new organizational strength from young leftists who normally would have gone into the Communist Party but who instead went into the revitalized Marxist wing of the Socialists.

"We have been trying to deal with the Communist phenomenon in France in part by draining it of its new strength," one Socialist Party strategist admitted in a moment of candor. "You will always have a Communist Party in France. The question is whether it will be weak or strong. Stalinist or moderate. Just keeping it in the ghetto, as the Gaullists and the Americans want to do, will never encourage the more moderate wing of the party to come out on top."

But the Communists seemingly chose to move back into the political ghetto last month by pushing negotiations on an updated version of the Common Program to the breaking point. They have bitterly attacked the Socialist leaders, and particularly the party's secretary general, Francois Mitterrand, for allegedly preparing to strike a deal with Giscard for a center-left government after the elections.

The criticism had indirectly resurrected the image of Mitterrand as the Proteus of French politics, constantly changing shape as political circumstances alter. Mitterrand, who comes from an affluent and religious family, served both as colonial minister and interior minister during the worst days of the Fourth Republic. It was his willingness to oppose de Gaulle, rather than any fervid commitment to socialism, that made him an important figure in the 1960s. Feud Over Platform

BOTH PARTIES have suspended political cooperation while they feud over the terms of the joint campaign platform. Despite the bitterness of their mutual attacks, the two groups could patch together a limited electoral pact for the March balloting under which they would agree not to oppose each other in runoffs.

But enormous damage appears to have been done to the once-strong prospects of a united Socialist-Communist coalition holding a working majority in the 490-member National Assembly and forcing Giscard to appoint a cabinet including Communists. The arithmetic of Assembly seats appears to have replaced ideology as the [WORD ILLEGIBLE] in the immediate future by providing Giscard [WORD ILLEGIBLE] room to maneuver.

Public opinion polls conducted for some of [WORD ILLEGIBLE] and for the Ministry of the Interior suggest that the Socialists will emerge as the largest party in the Assembly with about 160 seats. The Gaullists should fall from their current 173 members to between 120 to 140. Giscard will be able to muster slightly over 100 centrists and independents, while the Communists should win about 75 seats.

With no clear majority for any group and a projected loss of electoral authority for the Gaullists, Giscard should be able to choose between trying to keep afloat a technocratic cabinet based on his current prime minister, economist Raymond Barre, or making a serious effort to split off the moderate wing of the Socialists and forming a center-left government with them.

Moreover, Giscard is reported by French political sources to be readying a major change in the electoral laws that will do away with the present winner-take-all district voting for the Assembly and bring in a proportional representation system that would not come into effect until the next Assembly elections, which would normally be held 5 years after the 1978 balloting, but which could be called much sooner.

Whoever wins in 1978 "will have to face up to the fact that the 20 years of prosperity we had until 1973 are over," says Socialist Michel Rocard. The next government "will have to digest an average-sized economic crisis, at the least, and try to survive in that curcial first year."

Unions, which have held back on wage demands over the past year in part to provide a calm period that would help the left in the elections, will be under no such constraint after the March voting. And bills the government is running up in order to boost its chances of reelection will fall due soon after the balloting.

Giscard has chosen to run a budget deficit, something he solemnly promised not to do, just to keep the state machinery running this year. He has cut back sharply on government investment and decided to accept a meager 3 per cent growth rate for 1977 by deferring strongly inflationary measures until after the March election. He has made no serious moves to resolve the rising level of unemployment, which is being boosted by a draconian holddown on the imports on which French industry depends for growth - a policy designed to keep the trade balance and the franc stable in the short run.

"It is better to win with an empty will than to lose with a half-full one," asserts one of Giscard's political admirers. "Either the Gaullists or the Socialists may be in a position to bring down the government that he forms out of the March returns, but you have to wonder if they will dare accept that responsibility and condemn France to a new round of ideological warfare."