TBILISI, U.S.S.R. -- Noticing a guidebook in English tucked under a stranger's arm, a balding, bearded Georgian stops on the street and breaks into English:

"American? Tourist? You have come to see Georgian town, not Russian town. Good, good. Very different here," says this teacher, strolling and chatting as amiably as would an Italian in Rome who wanted to practice his English by making small talk about his country and mine.

But a few steps farther along Rustaveli Avenue, the teacher suddenly falls silent and remembers a previous appointment across town when I ask him to translate the writing on three posters in display cases facing the avenue.

They show cartoon drawings of fierce Afghan tribesmen, daggers between their teeth and dollars stuffed into their turbans, killing innocent civilians. Porcine U.S. Army officers and bankers devour gold coins as U.S. citizens starve.

What do Soviet citizens really know and think about Americans? For all that is different here in the sunbaked and dusty streets of Tbilisi, where Europe ends and the Middle East and Asia begin, answering that question remains as problematical as it does elsewhere in the Soviet Union. A chance encounter with an English-speaking teacher probably actually occurs by chance rather than by sinister design. Georgian teen-agers, rather than the KGB, keep you under surveillance as you leave your hotel. They sidle up within a few blocks to ask discreetly if you want to change dollars on the black market.

But even in this more relaxed atmosphere, relentless official propaganda about the iniquity of the American MIC (military-industrial complex) and an obsession with secrecy can shut down human contact and attempts at understanding very quickly. The teacher's aversion to describing posters hanging on (what I later discovered to be) a military office building is only one reflection of that.

There are other signs that the much touted new openness of Soviet society has done little to change the way in which Soviets view American society. One of the clearest signs for me is a certain stubborn refusal to be surprised by any of the unsavory acts that have been brought to light by the Iran-contra hearings in Washington.

To the extent that it can be successfully decoded and put down in synopsis form, the true Soviet reaction to the Iran-contra affair seems to me to run something like this:

Surprised? Not at all. This is what we have been told all along. Of course there were secret plots hatched in the Situation Room of the White House that were the real policy, while Congress put on a big show about being the voice of the people and the source of laws. We know all about making showcases of parliaments and constitutions, while the dirty work gets done somewhere else.

And we have had our own experiences with show trials, or what you call congressional hearings, when somebody other than the leader has to take the blame. You have a particular American twist, with the instant rehabilitation of the purged officials by television audiences. We usually wait several decades and lots of party congresses, but we know how impatient you Americans are.

So let us not waste time with talk about the self-correcting mechanisms of the American system, which turns out to be pretty much like ours.

The self-interest in such commentary by Soviets would be, of course, palpable. But Col. North and Vice Adm. Poindexter did all they could, in word and deed, to confirm the stereotypes of Americans that Russians and others abroad hold. This will be among the most lasting contributions of the Iran-contra gang.

That was brought home to me on the airplane from Tbilisi back to Moscow not by anything said by a Russian, but by something written by Edward L. Rowny, the retired general who is special adviser on arms control to President Reagan and Secretary of State Shultz, in an article that he published in the spring issue of The Washington Quarterly.

I had put the article aside to read in conjunction with arms-control interviews I had scheduled in Moscow. But with the final stages of the congressional hearings and the Russian refusal to be surprised by them very much on my mind, I found these remarks by Rowny to define, inadvertently, the great damage wrought by North & Co. abroad:

"Our open society ensures that our programs are consistent with their stated intentions. This is in contrast to the U.S.S.R. . . . "

The Russians, reading about the colonel shipping arms to the ayatollahs with Reagan's express consent, say they are not so sure, general.