It must have been something like Camelot: Massachusetts Sen. Edward M. Kennedy presiding as an Irish king, a commanding figure in tunic and tights wearing a crown of laurel, as befits a 56-year-old birthday boy.

Among his guests Saturday night at Castle Kennedy (otherwise known as his McLean estate) were a half dozen Cleopatras, including daughter Kara, sister Jean Kennedy Smith and sister-in-law Ethel Kennedy, whose arrival by chariot was planned as one of the party's more innovative entrances.

Other celebrants included a frequent but unidentified companion from Los Angeles known only as Melissa, in a flowing gown and a laurel crown to match her host's. She brought along many of the others' costumes and won rave notices from Kennedy family members for her creative ideas.

Invited to portray the historical political figure of their choice, other guests were creative as well. Broadway's Adolph Green even came as Eleanor Roosevelt's maid. (His costume was reportedly borrowed at the last minute from the hotel where he was staying.)

In kingly fashion, Kennedy's front door was protected by a moat filled with dry ice that sent up eerie wisps of mist. Entry was by specially built drawbridge, which two guards raised and lowered.

Kennedy's office called the party "private" and had no comment on either details or guests.

In Boston yesterday (his actual birth date) Kennedy's public birthday party had an added element of election-year fund raising. It was arranged by Kara and Teddy Kennedy, his children and comanagers of his reelection campaign. They're soon to be joined by their brother Patrick, a Providence College student who has filed as a delegate for Massachusetts Gov. Michael S. Dukakis.

Jazz great Dave Brubeck seemed to know something the White House didn't -- that its historic Steinway concert grand piano with the gilt eagle legs, affectionately known in Steinway circles as "The 300,000," is the spitting image of one in Leningrad's Philharmonic Hall.

"This is the largest piano ever made," Brubeck said of the 12-foot White House Steinway after he and his quartet entertained the nation's governors in the East Room Sunday night. "There's only one other like it in the world. It's in Leningrad and a twin to the one here."

Afterward, Brubeck told Arkansas Gov. Bill Clinton, who plays jazz saxophone, that he first learned of the Leningrad "twin" when he was on a concert tour in the Soviet Union last year. Russell Gloyd, Brubeck's conductor and manager, corroborated Brubeck's story, one Clinton said he also had heard.

Yesterday, however, a Steinway & Sons spokeswoman said the Leningrad "twin" is news to Steinway.

"It could have been a piano that belonged to a czar but it couldn't have been a twin to the White House piano," said Amy Blumenthal.

The only "twin" in existence is "The 100,000" -- so named because it was the 100,000th Steinway made -- and it is on display at the Smithsonian. Presented by Steinway & Sons to the White House in 1903 when Theodore Roosevelt lived there, "The 100,000" was replaced by "The 300,000" in 1938, when cousin Franklin was president.

Now, 50 years later, Steinway is about to unveil "The 500,000" to celebrate the firm's 135th year in business. Scheduled to debut June 2 in New York's Carnegie Hall, this concert grand is under wraps in the Rochester, N.Y., studio of Wendell Castle, who's been commissioned to create the piano's case. One unique touch will be the more than 900 names of living Steinway artists, including Vladimir Horowitz, Rudolf Serkin, Elton John and Dave Brubeck, laser-burned underneath the lid.

Blumenthal said that other than a world tour, plans for "The 500,000" will be announced at a March 3 press conference.

"It may be auctioned off," said Blumenthal.

And the minimum bid?

If one can judge by what's in a name, how about $500,000?

"It's my 10th time here for the governors' dinner -- and I've been here about seven other times -- so I don't think about it very much."

-- Kitty Dukakis on the White House, which at least her husband, Massachusetts Gov. Michael S. Dukakis, was busily checking out Sunday night even if she wasn't.

"You get my last name right and I don't care what you call me."

-- Virginia Gov. Gerald L. Baliles, vice chairman of the National Governors Association, on being identified as "Gary" Baliles by President Reagan Sunday night.

He came on board Feb. 1 but he wanted a month to get his "river legs." Yesterday, overlooking the Potomac from his brand-new vantage point as president and chief executive officer of the John F. Kennedy Center for the Performing Arts, former Time Inc. executive committee chairman Ralph P. Davidson and his wife Lou were officially welcomed by the center's board of trustees and his predecessor Roger L. Stevens, who in July takes his new title of founder/chairman.

Among the welcomers were the ambassadors of Switzerland, Cyprus, Israel, Ireland, Mexico, Malaysia and the Netherlands and a host of other Washingtonians, including former Democratic Party chairman Robert Strauss who, like Lou Davidson, is a Lockhart, Tex., native, a claim not many people can make.