"The descendants of James Fraser Gluck said they intend to be in touch with the library soon," said Matthew Weigman, a Sotheby's spokesman. He said the Gluck grandchildren had been told by Sotheby's that its research showed that the manuscript might at one time have been "located at the Buffalo library." Like an Overdue Book

William H. Loos, curator of books at the Buffalo library, said yesterday that the discovery might be described as nothing more or less than the resurfacing "of an overdue book." He said he suspects Mr. Gluck, a major benefactor of the library who was instrumental in persuading Twain to donate the Huck Finn manuscript, borrowed half of the manuscript and forgot to return it.

"Mr. Gluck must have taken that part of the manuscript home with him, presumably to read, and possibly forgot he had it," said Mr. Loos. "He died very unexpectedly and tragically at the age of 45 in 1897, 10 years after the manuscript had been presented to the library. Because there was no title page on it, just a pile of handwritten documents, it was probably simply swept up when his estate was settled, put in these trunks and nobody has looked at them all these years. We greatly honor the memory of James Fraser Gluck, the single most generous benefactor of this library. If he forgot to return this overdue book, we are prepared to forgive him."

The Twain manuscript was taken to California by James Gluck's daughter, Margo, a violinist who died in Los Angeles in 1957 and left the contents of the trunk to her nieces, one of whom is a librarian in Hollywood.

The handwritten Twain narrative includes an episode about Huck swimming out to a raft and witnessing a fight. The episode was removed from "The Adventures of Huckleberry Finn" and published instead as part of "Life on the Mississippi." A Theory Supported

"It proves the theory that this is an integral part of Twain's draft of Huck Finn and he did indeed extrapolate that and put it into 'Life on the Mississippi.' " Mr. Needham said. He said the incident was not in the original manuscript of "Life on the Mississippi," which is in the collection of the Morgan Library in New York. Mr. Needham was curator of printed books and bindings at the Morgan until last year.

The Gluck heirs brought the manuscript to the auction house in Los Angeles in October. "They brought it to us to authenticate what it was and to ascertain whether there was some potential for sale there," said David Redden, a senior vice president at Sotheby's. "We told them that before such a sale can go forward, we have to do rigorous investigation of the origin of this manuscript."

Mr. Loos said he was elated at the discovery. "It was always a fantasy of mine that someday the first part of the manuscript would surface and that somehow it would be returned," he said. "It is one of the greatest literary discoveries of the 20th century."