A well-known historic document dealer, who claims he once owned the James Whistler letters that ended up in the hands of art historian Charles Merrill Mount, yesterday disputed Mount's contention that it will be hard to prove that historic materials found in Mount's possession are actually the property of the Library of Congress and the National Archives.

Providing the most detailed description yet of how authorities expect to establish ownership of documents allegedly stolen from the library and archives, Kenneth W. Rendell, a Boston-based manuscript dealer, said the Whistler letters, for one, are well marked -- because he marked them.

"I sold those {Whistler} letters to the Library of Congress, and they have my handwriting and markings on them," said Rendell, to whom the FBI turned for help in identifying the letters of the 19th century artist.

Rendell's description of identification methods for the Whistler letters, as well as for those written by Abraham Lincoln, is the latest development in the unfolding drama involving Mount, who stands accused of possessing and transporting the documents, alleged to be the rightful property of the archives and library.

Mount, 59, was arrested and jailed twice this month after FBI agents discovered more than 200 historic documents in his possession here and in Boston.

The Brooklyn-born art historian and portraitist, who describes himself as "a perfectly normal Edwardian gentleman," has denied taking any of the rare letters. In a recent interview, Mount said the historical materials "were always mine and had been in my possession for 25 years."

But Rendell, who in 1983 exposed the so-called Hitler diaries as fake, said the Whistler and Lincoln letters recently sold by or confiscated from the art historian were never Mount's to sell.

"He seems to be a fascinating but very sad character," said Rendell, who called Mount "terribly naive" in believing he could sell the letters without questions being raised.

Rendell said he sold 40 to 50 Whistler letters to the Library of Congress between 1970 and 1974. Nine of those letters, according to a federal complaint, were among 25 historical letters Mount sold to a Boston bookstore for $20,000 on July 23, and which the Library of Congress later identified as missing.

According to Rendell, the Whistler letters he sold to the library had Rendell's handwriting on them in pencil, either at the top or on the back, a date or inventory number, a cost code, the price and a brief notation as to the letter's author and form.

"I went over them with the FBI, and I have the invoices on them, which are made out to the Library of Congress," said Rendell.

Another group of documents in Mount's possession, according to the complaint, included three Civil War era letters written or signed by Lincoln that are said to be missing from the National Archives.

Later, after FBI agents searched Mount's Capitol Hill rooming house and two bank safety deposit boxes he maintained under other names, the art historian was charged with receiving government property, specifically a letter by novelist Henry James that Library of Congress officials have identified as missing from their collection.

Rendell said the Lincoln letters also are easily identified because the contents of the Civil War president's letters were published in 1953 in an eight-volume set entitled, "The Collected Works of Abraham Lincoln."

"All collectors and institutions with Lincoln letters provided copies, and every known Lincoln that was in a museum, library or other institution in 1953 was in there," Rendell said of the published collection. "It's very easy to find out who has what."

According to a source familiar with the probe, the Lincoln letters found in Mount's possession are listed in the volumes of collected works.

Rendell said he has not seen the James letter that was confiscated in Washington. But he said two James letters found in Mount's possession in Boston carried distinctive markings indicating they had been torn from an old bound volume, leaving tiny holes in the left-hand margin where the letters had once been stitched into a book.

Mount yesterday called Rendell "a crackpot." Speaking specifically about the Whistler letters, he said, "They may have his code on them now, but there was nothing on them when I had them. Remember, they have been out of my possession since July 23."

A survey of about 25 of the 30 dealers who handle autographed materials and historic documents turned up only a tiny number who had heard of Mount before his arrest. And only a few of these, including Rendell and the Boston bookstore, said they had ever bought items from the art historian or sold him any.

One of these, Edward Bomsey of Annandale, said he had purchased 11 items from Mount, including letters signed by Theodore Roosevelt, Woodrow Wilson, Dwight D. Eisenhower and Rudyard Kipling, in four transactions, the first of which took place in May. He said he is still waiting to hear from authorities whether the materials are considered stolen property.

"I've given a full description to the Library of Congress and the National Archives, and if they can show me they're theirs, I'll return them," said Bomsey, who said Mount told him he had bought the items at an auction in England.

At one point, Bomsey said, Mount tried to sell him some Whistler letters.

"I wasn't interested, but I contacted the Freer Gallery, and they indicated the University of Glasgow might be interested," Bomsey said. But Mount, Bomsey added, "was reluctant to provide photocopies. He said he didn't want his items bandied about."

Mount, according to numerous art dealers, was a better known figure on the art scene, though he was dogged by persistent rumors of art forgery, stemming from a British newspaper article published in 1967. Mount sued for libel but said he could not afford to pursue the case once he moved back to the United States.

Mount, according to these dealers and others, also had a litigious nature, and frequently sued members of the art community, some of whom he accused of stealing material from his Sargent biography.

"He has a deep commitment to {John Singer} Sargent and considered Sargent his territory in many ways," said Patricia Hills, who organized a Sargent show last fall at the Whitney Museum of Art in New York. She said she deliberately avoided reading Mount's biography of Sargent until after she had written her own material.

"Anyone who has written on Sargent is aware Mount is around and that he might sue you," said Hills.

Mount's recent notoriety has had aftershocks at the public library in New Bedford, Mass., where Mount was asked in 1977 to authenticate a portrait of George Washington thought to be an original by Gilbert Stuart.

Mount said it was genuine, adding, according to newspaper clippings at the time, that "no one but an expert would realize that it's authentic." An official at the National Portrait Gallery pronounced it a copy, and the painting's authenticity has been in dispute ever since.

Said a library official yesterday: "Certainly, after last week, the portrait's status around here is more questionable."