The National Symphony Orchestra concluded its Beethoven Festival on Friday and Saturday nights at the Kennedy Center with a flurry of rarities heard only on recordings, if at all.

The Piano Concertos No. 4, played on Friday, and No. 2, played on Saturday, are standard repertoire, and the Concerto for Violin, Cello and Piano and the Romance No. 2 for Violin and Orchestra are performed occasionally, as is the Piano Concerto in D, transcribed by Beethoven from his Violin Concerto. They have opus numbers, indicating their approximate place in the chronology of Beethoven's compositions. But two works on the Saturday program had the WoO numbers (abbreviations for "without opus") that designate unfinished or unpublished music: a Concerto Fragment in C for Violin and Orchestra, WoO 5, and a Rondo in B-flat for piano and orchestra, WoO 6.

Conductor Leonard Slatkin, in remarks to the audience, referred to the "odds and ends" on the program, inevitable in a festival presenting "almost all [Beethoven's] works for soloist and orchestra." Some early works reflected both Beethoven's respect for Mozart and his eagerness to go beyond Mozart's style. But two pieces on the Friday program stood out as revolutionary innovations.

Piano Concerto No. 4 opens not with the usual orchestral ritornello but with a gentle, meditative piano solo. You might call it a cadenza or an instrumental recitative; by any name, it announces that this will be a concerto like no other. The piano arrangement of the Violin Concerto opens, like its model, with four mysterious notes on the timpani, which become a basic theme of the first movement. Beethoven wrote out the first-movement cadenza and (this is the revolutionary part) included the timpani in an intense dialogue with the piano, calling to mind music of Bela Bartok more than a century later.

The transcription mingles the joy of familiarity--very strong in most classical music fans--with the complementary joy of hearing a different treatment of familiar material. It may fascinate lovers of the Violin Concerto, but Bach did this sort of thing more often and more expertly than Beethoven. He also wrote better concertos for multiple soloists. In comparison, Beethoven's Triple Concerto is too often concerned with mere traffic control among its three soloists.

But minor Beethoven is still Beethoven. Both programs were enthusiastically applauded by the audience, which also applauded Slatkin's announcement that next year's Beethoven Festival will present Gustav Mahler's reworking of four symphonies and several overtures. This annual festival is not only a musical delight; it is a learning experience.

The orchestra generally played well, particularly considering the amount of unfamiliar material and its limited rehearsal time. The soloists, too numerous to discuss in detail, played with imagination and solid musicianship. They included NSO concertmaster William Steck, pianist Garrick Ohlsson and the Kalichstein-Laredo-Robinson Trio, with violinist Jaime Laredo and pianist Joseph Kalichstein playing as soloists as well as joining cellist Sharon Robinson in a smoothly integrated reading of the Triple Concerto. Also worth mentioning is the orchestra's new principal timpanist John Tafoya, who held a fascinating dialogue with Ohlsson in the cadenza of the Violin Concerto transcription.