The other night one of the village's black-robed Shiite Moslem elders strode into the tent at the checkpoint manned by U.S. marines here. He had a baby in his arms and a special request of platoon Sgt. Michael J. Murphy.

Would the sergeant be so kind as to summon a black marine to kiss the baby? The request surprised Murphy but not Warren MacFarlane, the black marine called to perform the honors. He had learned from another machine-gunner about the local belief that it was good luck for a baby to be kissed by a black marine.

"He said he kissed a couple," MacFarlane said.

For the platoon of marines, posted for four months at the southern entrance of this bomb-ravaged Shiite village across from Beirut International Airport, there is little that is truly startling anymore.

As they packed up to return home--and be replaced today by a new wide-eyed contingent--the veterans also recalled how they had learned to contain initial concerns over Israeli soldiers' reconnaissance-by-fire operations, conducted about 300 yards away each morning at 7:30 for the first three weeks of January.

"One thing I can say about the Israelis, they're punctual," said Murphy, 30, of Falls Church, Va. There are a couple of bullet holes in the roof of one of the marine tents from ricocheting Israeli rounds. Other stray shots landed near the outhouse, he said.

"It's a good thing about being a human being; you can adapt to anything," Murphy said. "After the first couple of times, nobody got alarmed about it anymore. We'd put on our gear. Get behind the sandbags. Take up positions."

They learned to sit back and watch through binoculars the shellings and machine-gun exchanges by Moslem Druzes and Phalangist Christians in the hills about 1,200 yards from the marine position, although many of the Americans thought they should be up in the hills putting an end to the fighting.

The marines' main complaint is that the policy makers in Washington have set too strict a limit on the mission here.

"What it boils down to is that this is against our training," said Murphy. "We're an offensive combat unit, but here we've been in a static defense. There's not much we can do about the things that go on around us. I suppose that's because of the public sentiment back home."

Pfc. James D. Posey, 20, of Manassas, Va., was more blunt: the policy makers "treat us like boys and we're not. We're goddamn United States Marines."

The marines at Checkpoint 51 have had the closest contact with Lebanese. Most of their counterparts are confined to fixed positions along the airport perimeter. It is self-contained, an existence similar to life at any U.S. military post.

Fatigues are laundered aboard the transport ships offshore and two hot meals are prepared there each day and helicoptered to the men. The third meal is the newest version of military rations: "MRE," meals ready to eat, an updated version of C-rations and easily recognizable by veterans of other eras. At night, there are American movies on Betamax.

Although French and Italian soldiers in the multinational force initially were given liberty in Beirut and were familiar presences at its cafes, the marines have seen the capital only by joining organized tour groups. There have been similar outings to historic sites in Lebanon and skiing trips to some of the more tranquil mountain villages.

This caution had initially brought titters from the international community over the "overprotected" marines. But the laughter halted after attacks on French soldiers. The Italians and French have cut back on the freedom they had given their troops.

The marines at Checkpoint 51 have not been the target of any deliberate violence. But even as they chafe at the restrictions, they express a keen sense that they are on shifting ground here.

Marines patrolling the village have been greeted by smiles and waves, and by pealing choruses of young voices saying hello and asking for the cocoa that comes in the marines' dried rations. The Moslem women sometimes offer a gift of fruit, smile, then quickly avert their eyes.

No young men were in the village when the U.S. contingent arrived four months ago, but lately they have begun to trickle back--and they are not friendly.

Mostly, the marines said, the young men confront them with hard, cold stares. Occasionally one yells, "Khomeini good." Last weekend, as marines patrolled a narrow street, a car with two of the village's young men--their radio playing rock music at full blast--swerved close to the Americans.

Suddenly the booming accents of Long Island filled the village street as Cpl. Robert Gioffi, 21, of Suffolk County, N.Y., turned and yelled, "At ease."

The young Lebanese men protested, "Why? why? " The confrontation quickly ended when one of the marines lifted his M16--which by regulations was unloaded--off his shoulder and held it at port arms for a few seconds. "It really freaks them out," said the marine.

Another change in local attitudes, small but irritating, the Marines said, has been noted among the small children who hang around the checkpoint and occasionally run errands into the village to pick up sodas and cigarettes. Lately, the marines said, some of the children have attempted to beat them on the exchange rate, apparently in the belief that the Americans have not kept up with the fluctuations in the value of the Lebanese pound. But mostly, marines stressed that relations were good--especially with the farm laborers nearby.

Some marines have picked up a little Arabic, Cpl. Donald Alvarez, 21, of Hofstra, N.Y., becoming the most proficient. He seemed to be the most reluctant to leave, saying villagers promised him a house, a job and a wife if he would stay. "These are my people," Alvarez is fond of saying.

Marines have made friends among the Lebanese Army soldiers who are ostensibly in control of Checkpoint 51, despite initial difficulties. One thing that annoyed them, marines said, was the way the poorly equipped Lebanese would finger and toy with the flak jackets and fatigues of the Americans. But what produced the most strain was the way the Lebanese soldiers attempted to follow the Arab custom of greeting by kissing the marines on both cheeks.

Sgt. Murphy said he went to the Lebanese commander and told him, "Sir, I wish you would instruct your people not to kiss my marines. They do not take it well."

The assessment of the marines at Checkpoint 51 is that the Lebanese soldiers with whom they work need a lot of training, better equipment and more discipline and need to be more thorough in their search of cars passing through the checkpoint. It angers the marines that many cars are waved through by the Lebanese soldiers without search.

"If it was up to us, we would be checking all cars," said Clayton Miller, 21, of Forest Hill, Md.

"They tell us we can't. They say we're here to back up the Lebanese," John S. Leigh, 29, of St. Louis, shrugged.

Last week, one car the Lebanese did stop turned out to have a couple of magazines of the ammunition used in Soviet-made AK47 rifles. Marines said they were concerned that the Lebanese soldiers, feeling proud of their discovery, did not look further.

The plan to have the marines serve as backup to the Lebanese frayed when Israeli soldiers began their reconnaissance-by-fire and then sent tank patrols near Checkpoint 51. "When the tanks started coming down the road, the Lebanese soldiers got a little tense," Murphy said.

Later, an agreement was reached that if there was a confrontation, or more serious trouble, the American staff sergeant would be in charge of handling it, giving orders to the Lebanese captain and his men.

The marines said they do believe their presence has brought a period of relative calm to people who have endured years of war. But they have found the complexity of the factions and disputes here baffling.

Murphy pondered the histories of Lebanon he read before coming and talked about the posters showing Iranian Moslem leader, Ayatollah Ruhollah Khomeini, that started appearing in the village a few days ago. He had one of the Lebanese lieutenants translate some of the Arabic on the poster. It said, "We must unite under one God."

But, Murphy observed, underneath the message and the likeness of Khomeini were drawings of dead people lying around tanks. "It's kind of brutal," he said. Later, he added on the strife in Lebanon, "If you think about it, you can't see any end to it."