WEBER'S UNFINISHED opera Die drei Pintos is of greater interest in the context of Gustav Mahler's career than that of Weber's own. It was this work, which Weber left in the form of sketches for an overture and only seven vocal numbers, that the 26-year-old Mahler fleshed out with material of his own and from Weber's other works in 1886 - when he was also conducting a Weber opera cycle in Leipzig in observance of the centenary of Weber's birth.

In addition to the fleshing-out, Mahler of course orchestrated the entire work and he also rearranged the sequence of the original Weber numbers (omitting the overture) without, however, altering the naive but charming plot (Theodor Hell's adaptation of a novella by Carl Ludwig Seidel, in which two young men successively assume the identity of the actual Don Pinto, who is, in the great tradition of such works, betrothed to a beautiful young woman who has never seen him). While involved in the project, Mahler also became involved with the wife of Weber's grandson, and the affair nearly had serious consequences - but that is another story.

What is more to the point is that Mahler so absorbed Weber's style for this unlikely undertaking that the finished product is quite seamless; no one has ever been able to guess where Weber leaves off and Mahler begins. We may still have a long wait for a production of Die drei Pintos on an American stage, but the opera has been recorded now, and it proves to be not only seamless but enchanting, reminding us, among its many attractive points, that Weber was a contemporary of Rossini as well as of Schubert.

The performance offered in the new three-disc RCA set (PRL3-9063) has all the polish, wit and all-round infectiousness such a work needs to make the strongest impression. Gary Bertini conducts the Munich Philharmonic, with the Netherlands Vocal Ensemble and a stunning arrary of soloists - Lucia Popp, Werner Hollweg, Hermann Prey, Kurt Moll, Heinz Kruse, Franz Grundheber, Kari Loevass, Jeanette Scovotti. They all seem to believe in the work, and to be enjoying themselves grandly. The sound, too, is first-rate.

And that is far from all, for the documentation with this release is a most valuable and distinguished piece of work in its own right. Jack Diether, who has added to the general Mahler lore in his probing and perceptive annotations for so many recordings, has given us no mere outline or sketchy background note here, but has provided a very thorough history of the work, its fate during the six decades between Weber's death and Mahler's involvement, and precisely what Mahler did.

Not the least part of Diether's contribution is an itemization of the 22 sections of the work, showing which were composed by Weber for this opera, and the specific sources among his other compositioons for the portions adapted or composed by Mahler. And even this feat of scholarship seems to exude the same sort of happy enthusiasm as the performance itself.

In every respect this is an outstanding production; it is probably an important one, and without question a delightful one. Jaded opera lovers should find it especially refreshing, but one needn't be either jaded or especially opera-oriented to enjoy it.