It was as close to a mob scene as you might find at the genteel Library of Congress. Three lines of people, 300 or more, wound through its corridors waiting an hour or longer to buy first-day postal covers featuring a new 20-cent Library of Congress commemorative stamp and to get them autographed by Postmaster General William F. Bolger and Librarian of Congress Daniel J. Boorstin, among others.

For the philatelically uninitiated, a first-day cover is an envelope containing a brand-new stamp, carefully canceled so as not to obscure its design and containing the legend, "first day of issue." On that first day, it is issued from only one post office--in this case, Washington.

The stamp, in red and black, is based upon a photograph of the main library building, now known as the Thomas Jefferson Building, taken in 1898 when it was just one year old.

Covers sold at the library yesterday were prepared by the Library of Congress Philatelic Club, which proudly labeled them as its "cachet number 1." On each envelope was a handsome drawing of the library facade.

The club had prepared 1,500 covers for sale yesterday, but the supply was depleted so quickly that they were soon rationed, one to a customer, with buyers invited to place orders for future delivery.