A Style section article yesterday incorrectly identified Noel Black, director of international public affairs for the Amway Corp.

"Is very musical gift," said Mstislav Rostropovich, holding a brightly wrapped package to the ear of Leonard Silverstein and shaking it gently. "You can tip it and hear the music, glug, glug, glug, glug, glug." What had begun as a farewell party for the National Symphony Orchestra (which flies to Zurich today to begin its European tour) had turned into a belated birthday party for the orchestra's president.

"We all wish you 100 years more," Rostropovich told Silverstein, who observed his 60th birthday last week, "and stay our president for next 100 years."

A symphony orchestra by itself can make a fairly large party, and with friends, board members and officials of Amway Corp., which is paying the expenses of the European tour, the NSO packed the Kennedy Center Atrium after its final pretour concert Friday night. Guests at the party included President Reagan's national security adviser, William Clark, Secretary of Defense Caspar Weinberger, newly arrived French Ambassador Bernard Vernier-Palliez and Amway president Noel Black, who gave everyone going on the trip a canvas under-the-seat bag full of Amway soap products. "We noticed that there will be seven consecutive days when the orchestra will be unable to do its laundry," Black said.

Orchestra members seemed unconcerned about the tight schedule for the tour, which covers 16 cities in 26 days. "Compared to the way I work in Washington, it will be easy doing one-night stands all over Europe," said percussionist Tony Ames, who also manages the 20th Century Consort and the Millennium chamber music organization.

"I'm bringing my best tympani -- my Strad, so to speak," said tympanist Fred Begun, explaining that musicians usually bring a second instrument on tour and leave their best one safely at home. "All of the orchestra members are bringing their prime instruments, and we're expecting prime results."

In a quiet corner, two members of the string section chatted about an absentee. "Pooks will definitely be there in Zurich to bark at us when we start our first rehearsal," said one.

"Good," said the other, "we couldn't rehearse without Pooks barking."

"Who was it he always used to bite?"

"Pooks doesn't bite; he only barks."

Actually, as an NSO staff member clarified, Rostropovich's dog Pooks is not a "he." She will be in Zurich Thursday.

Meanwhile, Weinberger was discussing exercise with Rostropovich. "I don't think jogging is as good as conducting," Weinberger said. "Keep on conducting and you'll have a long life."

"Now you know why I take up conducting -- for long life," said Rostropovich.

The French ambassador , like many others at the party, was excited about Gershwin's "Promenade," the encore in Friday night's program, which included a very jazzy clarinet solo by Loren Kitt. "The audience reaction reminded me of the Jan. 1 concerts in Vienna, when they play their own Viennese music and the people love it," he said. There was diplomatic unanimity on the subject; across the Atrium, Dutch Ambassador Jan Hendrik Lubbers was telling Ann Vanderpool of the NSO board of directors that the orchestra should play more Gershwin (who is very popular in Holland) during its concerts there.

National security adviser Clark, in a brief speech, gave the orchestra unofficial diplomatic status. "You know," he said, "the United States has 144 ambassadors abroad. I think I can say for the White House and the State Department that we very much appreciate Amway's selection of the 145th -- Slava and his orchestra."

Besides the liquid gift from Rostropovich, which turned out to be a crystal decanter full of 100-year-old cognac, NSO president Silverstein received gifts from the orchestra staff (a Russian lacquer box) and musicians: a digital wall clock showing the time in the world's various time zones "so that you can keep track of where we are." And a special accolade from Rostropovich: "I don't agree with his last name, Silverstein. For me, that name is at least Goldstein," he said, engulfing the NSO president in a bear hug.

Rostropovich also led the guests in a chorus of "Happy Birthday to You," after which executive director Henry Fogel said, "now you can all say in your re'sume's that you have sung under Maestro Rostropovich."

In return for all the birthday presents, Silverstein and his wife, Elaine, gave a gift to the orchestra. "We have heard that some of you are not very comfortable on stage," she said, "and we are giving you new chairs so that you will sit more comfortably and the music will sound even better." The orchestra gave a standing ovation.