It all started when an anxious guest at the Mayflower Hotel early this week lost the key to his safety deposit box and forgot which box in the hotel's vault was his.

The hotel's own record on the box was missing, too. Security men came in to help, opening several boxes in search of the right one. Along the way, they chanced upon a dust-covered one that apparently had been unused for years. Stunned, they discovered it was stuffed with several sheets of crisp uncut sheets of U.S. currency dating back to the 1920s and 1930s.

A quick check showed that the box belonged to one-time Washington socialite Evelyn (Evie) Walker Robert, a permanent resident at the hotel for many years whose husband, Lawrence (Chip) Robert, had been an assistant secretary of the Treasury during the Roosevelt administration almost half a century ago. Both Roberts are now dead.

The Mayflower called in the Secret Service. Agents hauled away the 13 sheets of assorted $1, $2 and $5 silver certificate bills for examiniation.

Yesterday, Richard E. Keiser, special agent in charge of the Washington held office of the Secret Service, declared the bills genuine.

The face value of the 216 bills is a nominal $324, but Treasury officials estimated the value to collectors at $8,000 to $10,000. b

"At first, we thought they were counterfeit," Keiser said, "but they're genuine, they're good, all right."

Agents, he said, still are trying to determine through the serial numbers on the bills whether they were stolen or purchased legally. "We may never know," Keiser said. Uncut sheets of U.S. currency were available for purchase by the general public until 1954, according to the Bureau of Engraving and Printing.

Chip and Evie Robert, both popular figures on the Washington social scene for decades and known for their eccentric moments, kept several of the uncut currency sheets around their Mayflower apartment for years, according to both Mayflower employes and family members.

"They were like a gimmick, something they'd show to people," said long-time Mayflower bell captain Pete Flaherty yesterday.

"They had a lot of fun with them. They were always playing some kind practical joke," said Hazel Robert, widow of Chip Robert's brother, Pierce, in Atlanta.

Secret Service agents said some Mayflower employes recalled Evie Robert occasionally cutting out single bills from a sheet of bills to tip bellmen.

Family members and Mayflower executives said they do not know why the currency sheets remained in the safety deposit box after the Robert's deaths. Evie Robert died in 1972 and Chip Robert in 1976.

Mayflower assistant general manager Michael Horrocks speculated that the executors of the Robert's estates simply were unaware of the safety deposit box. "It's happened that way here in other cases," he said.

Chip Robert was assistant secretary of the Treasury from 1933 to 1936. He also served as treasurer and then secretary of the Democratic National Committee in the late 1930s.

Horrocks said discovery of the uncut currency sheets was an unexpected by-product of the hotel's search for the safety deposit box belonging to the guest who had lost his key.

The guest was unable to pinpoint his box among the approximately 75 safety deposit boxes in the hotel vault adjacent to the front desk in the main lobby, Horrocks said.

Hotel security men then began drilling out the locks of several unmarked boxes, he said, "and that's when we came across the money."

Keiser of the Secret Service said the currency is valuable to collectors not only because of its age but because it is in sheet form. The currency consisted of 10 sheets of $1 bills with 18 bills on each sheet (1935 series), two sheets of $5 bills with 12 bills each (1934 series) and one sheet containing 12 $2 bills (1928 series). The sheets had been neatly folded or rolled to fit into the safety deposit box.

Keiser said the Secret Service would retain custody of the currency while the matter is under investigation.

L. W. Robert IV, grandson of Chip Robert, said in a telephone interview from Atlanta that the family had been notified by the Secret Service of the discovery.

"We will be getting together with the executors of the estate," he said. "It will all be handled properly."

Even though the bills are comparitively rare silver certificates -- as distinct from the more common Federal Reserve notes in circulation today -- they are worth only their face value, Keiser said. "If you try to use (one of the Roberts' $5 bills) at the Safeway," he said, "you're still only going to get $5 worth of groceries."