"My sister and I both had our weddings in that back dining room," recalled Elizabeth Cox, one of the former residents of the Gruman-Cox estate, site of last night's 1980 Decorator Showhouse preview party. Both Cox, her sister, Mary Beauregard, and their spouses were present at the private unveiling of the 31-room house off Chevy Chase Circle, newly decorated by 31 Washington interior designers.

Both sisters loved the house's new look. Archibald Cox, Elizabeth Cox's husband and former Watergate prosecutor, agreed for the most part, but wistfully said, when he reached the lavish spread under the big-top tent behind the house, "Did you see what they did to the pool? It's all filled in. I guess I never used it much anyway."

The annual preview party to benefit the National Symphony Orchestra was well attended, 520 guests at $60 apiece, according to Laurie Naylor, the party's chairwoman. Rumors abounded that the house had been sold that night to the Bulgarian ambassador, but no one could confirm it. "I do know that he called last minute today to ask if he could attend," said Naylor, "but we had to turn him away. We were already oversold by 70." The house has been on the market for a half-million dollars. o

The ambassador of Great Britain, Nicholas Henderson, and Lady Mary Henderson were the preview party's honorary patrons, although only Lady Henderson attended; the ambassador was in New York with Prince Philip. In honor of the British sponsors, the theme of the party was Scottish, complete with the seven-man Washington Scottish Pipe Band, and tartan banners lent by the St. Andrews Society to decorate the buffet tents.

Other guests included NSO president Martin Feinstein, University of Maryland President John Toll and NSO board chairman Austin Kiplinger, whose wife, Mary Louise, was properly attired in a red and black tartan plaid dress.

A scotch-tasting was set up in the portecachere or carport, supposedly to leave partygoers with a taste of Scotland when they left. But the tasting got underway long before it was time to go home.