A nine-year-old impresses America -- 216 years later; Clearly an early work -- but such imagination, poise, and mastery!

It seems hard to imagine actually experiencing an unknown work by Mozart for the first time. Yet that is exactly what the Mostly Mozart Festival in New York has just allowed music lovers to do.

For more than two centuries scholars have assumed that an early F-major symphony existed only in a sketch of the first 15 measures, penned by Mozart's father, Leopold, on a newspaper wrapper. Last Feb. 12, the Bavarian State Library announced that an entire set of orchestral parts in Leopold's hand had been discovered in a private (and still anonymous) collection. Then it was a simple matter to reconstruct the score, already known as fragment K. 19. Not surprisingly, this complete work is now numbered K. 19a in the complete Kochel catalog of Mozart's entire output. MAy 17 was the date of the first performance since 1765 or so (in Europe), and the American public premiere took place at the Kennedy Center, preceded by a performance at the White House lawn. (See adjoining article on this page and the Monitor arts pages of June 4.)

When New York heard the work, all the performances were conducted by Leonard Slatkin. Though clearly an early work, it is just as clearly Mozart in the making. Already in the first movement the sort of melodic inspiration that marks the composer's maturing years is to be heard, if without the almost dizzying invention and variety of later compositions. The second and third movements of a work lasting just under 12 minutes are not quite in the same league, but to think that a nine-year-old could write such an accomplished and imaginative piece, with such poise and thorough mastery, underscores the obvious -- that the world has yet to know another musical genius the likes of Mozart.

The conductor sits at the harpsichord, as was the custom of those days -- a custom that Mozart, as well as his teacher Haydn, was to abolish in the none-too-distant future. That harpsichord part echoes, supports, and fills out the orchestral writing -- an orchestra of strings, two horns, and two oboes. Mr. Slatkin delivered a performance of exceptional persuasive suaveness and brio , and his work at the harpsichord proved exemplary.

The rest of the concert was of unusually high quality. Slatkin's account of the C-major Symphony No. 36 (K. 425) was lucid, graceful, and handsomely paced. His partnership with Alicia de Larrocha in the C-minor K. 491 piano concerto (No. 24) made for a particular engrossing account of this stormy, melancholy work.

Miss De Larrocha is an incomparable Mozartean, particularly in slow movements , where her delicately interior style points to depths of emotion and modd that a more vigorous or flashy approach would betray. This sort of musicianship is becoming increasingly common at mostly Mozart, which is why it is now in its 15 th season of often sellout success.

The F-minor symphony will be offered again July 27 and 29 under the direction of piani st-conductor Christoph Eschenbach.