When "Mr. Leon Libofsky" was introduced as a member of the Soviet-American Trade Commission for a speech before some 30 journalists at a private dinner at Mel Krupin's Thursday night, he seemed pretty authentic. But when the journalists, who had all been on an unofficial trip to the Soviet Union, were told of the "imperialists and criminals . . . that run your country," things did not seem be adding up.

"Chernenko's cold has been cured," said the "Soviet" in halting English. "He's now permanently ensconsed in the walls of the Kremlin. And Premier Gorbachev is fun loving. He drinks Scotch, he loves jazz . . . and has a wife that does not dress in potato sack . . . Gorbachev will not be pushed around by gangster Reagan and his coterie of bomb throwers . . . " At that point, most people began to get the drift.

The "Soviet" was Central Casting actor Irwin Ziff, hired by USA Today editorial director John Seigenthaler and Chicago Tribune editor Jim Squires to pull a fast one on their friends. "Some of them began to get the idea this wasn't exactly kosher," said Ziff, who said he had met with Squires and Seigenthaler to go over the script beforehand. "But some of them never caught on."