While 250-old Washingtonians were watching Rachel Roberts in "Yanks" at the American Film Institute last night, the Welsh actress was upstairs watching herself on televison in "The Sorrows of Gin" -- the first of the three John Cheever stories.

The television show (which she loved) ended in time for her to catch the end of the movie (which she also loved), take a quick bow for an enthusiastic audience when George Stevens introduced her, and join the members of the AFI Film Club for an apres-cinema buffet at the Madison -- the first of five such events this season for which members pay dues of $175.

At the buffet, Ina Ginsburg, who coordinates the program hesitated over whether the season opening last night was the sixth or seventh. She finally decided it was the seventh, and Sen. Charles Percy (R-III.), who was dining beside her, commented gallantly that "she begar when she was 19."

Across the table, Stevens and Roberts were reminiscing about their first meeting -- in Moscow in 1965, at a memorable film festival during which a Russian guide mistook Rex Harrison for Averell Harriman. Stevens recalled worrying about hidden microphones in his hotel suite, peeling back the rug, finding a large wire, cutting it and then enjoying "the best night's sleep I had in Moscow." $"tThe next morning," he said, "my interpreter asked me if I slept well, and I told her I had. 'I am glad,' she said. 'Last night at midnight, the chandelier fell down in the ballroom directly under your bedroom.'"

Meanwhile, Ginsburg was lamenting to Percy that ever since she gave him a contribution, she has found it impossible to get her name off Republican mailing lists.Percy replied with a brief oration on why he opposes the use of Social Security numbers for general identification purposes: "Once you get in the computer, you know, there's no escape."

The only exception the Senate allows on its rule against the use of Social Security numbers, he said, "is Sen. Russell Long, who ues them to trace runaway fathers. He recovered a billion dollars that way last year, and now we call him 'Billion-Dollar Long,' You know what Sen. Dirksen used to say: "A billion here, a billion there, and pretty soon you're talking about real mney."

For AFI, real money is the $50,000 it nets each year from its Film Club program -- enough to cancel half the deficit between its $400,000 annual budget and its $300,000 in ticket sales.