The Andrew Young touring southern Africa last-week appeared to be a new man, sobered and seemingly far wiser about the complexities of southern Africa than ever before although still holding tenaciously to what he calls his own "irrepressible optimism."

In the view of British and American journalists, his own British diplomatic colleagues and the white leaders of Southern Africa, Young has matured from his earlier forays into Africa.

He has ceased making ill-timed and ill-considered remarks about the white nations and leaders the Carter administration must learn to deal with whether it makes their racial policies or noe.

He has learned to take a quiet back seat when that is his indicated role and even to avoid the limelight he so easily attracts to himself in this part of the world by virtue of his character, color and special position within the Carter administration.

Andrew Young was perhaps the biggest surprise to those accompanying or meeting him on his latest diplomatic journey into southern Africa where the reactions of all the other parties involved was pretty much as expected.

It was his first meeting with both South Africa's prime minister, John Vorster, and the Rhodesian leader, Ian Smith, and it was widely expected the sparks might fly after all the hostile comments Young has made about these two white-ruled nations. But such was not the case.

Instead, Young came out of his "confrontation" with Vorster and Smith amazingly well, with indications that both white leaders of Southern Africa found him worth courting rather than just condemning for his past remarks,

The Rhodesian leader said publicly he found Young and the entire American negotiating team "more pragmatic" than the British and also far less vindictive toward the white Rhodesian government. Privately, the Rhodesians said they were positively impressed by him, to their own surprise.

Young seemed just as surprised at how well his meeting with Smith went, and he was careful to avoid making any antagonistic remarks about him to reporters afterwards.

The attitude of South Africa's white leaders, whose government Young once described as "illegitimate," also showed signs of changing in the wake of his recent speech in the Nigerian capital of Lagos, where he spoke before an international audience against the imposition of sanctions on that white-ruled nation and against curing the disease of apartheid be seeking to kill the patient.

While Vorster reportedly prepared himself for dealing bluntly with Young, the prime minister's aides insist he was courteous and correct with the Black American ambassador with an insistence that suggested they want to learn now to get along with him.

Throughout the trip, Young carefully deferred to the British Foreign Secretary David Owen, allowing the higher-ranking British diplomat to answer most of the questions posed at news conferences and making no attempt to outperform him.

This was in keeping with the American approach toward the latest Anglo-American peace initiative over Rhodesia: that it should be seen as a British-led one with the U.S. government simply supporting it. The African frontline" leaders have made it known that this is the way they want it to avoid antagonizing the Soviet Union and turning thedelicate Rhodesian issue into an arena for East-West conflict.

As a result of this, journalists discovered at press conferences an Andrews Young who was subdued and deferential, a new side of the man they had not seen before. It was a role the daringly outspoken and dashing American diplomat hardly seemed cast for or willing in the past to play.

Nonetheless, most of the indications were that Owen and Young got along very well together. A slight suggestion that there was some tension between then brought the two stylish diplomats hopping back into the journalists' compartment of the British Royal Air Force plane to affirm vehemently that the opposite was true.

What followed was a session of mutual admiration during which they noted how nicely they complemented each other, Young being the moralist subject to emotional outbursts of broad principles during their talks with the white and black leaders of southern Africa, and Owen the meticulous lawyer (he is actually a general physician concerned about the fine details of the British-American proposals for a settlement in Rhodesia.

Perhaps the most common criticism voiced of both Owen and Young by outsiders watching them on their joint "mission impossible" was their seemingly unshakeable faith in the common sense and rationalism of all the parties involved in the complex Rhodesian equation, despite the sizeable body of evidence to the contrary.

"I'm an irrepressible optimist," Young told a press conference on the lawn of the former residence of the British commissioner in Salisbury a few hours after his three-hour meeting with Smith.

"I just think that when people are face to face with death, they come to their senses. I think that we are seeing entire countries facing destruction and chaos and that's usually a sign that before long they will come to their senses and do something other than fight," he said.

But most local observers of the Rhodesian scene have long ago come to the conclusion that the kind of rationalism and common sense Young is talking about simply do not exist because the issues at stake are vital ones of power and survival for a besieged white minority as well as rival black leaders.

In their latest proposals, the British and Americans seem to have tried to wish away the problem Ian Smith by asking him to "surrender" power long before he perceives himself defeated. This may not be an altogethr "rational" approach on their part, either, Smith, after all, has just won a massive vote of confidence at the polls from the 270,000 whites living in Rhodesia finish weighing all the risks of "facing the chaos" ahead of them if they go alone against those of the proposed settlement under British and international auspices, they will eventually choose the latter.

This remains very much to be seen.

In the meantime, Smith's reply is that the Anglo-American proposals, such as they stand, promise to bring even greater chaos than his internal solution and he apparently still has the vast majority of whites behind him in this conviction.