IMAGINE A resolute musicologist combing the dank archives of some Alpine convent and coming upon a faded score that, once microfilmed and authenticated, turns out to be an unknown Mozart opera. It is an early comedy, and it is a gem. The subject is one he would get back to in a few years: the topsy-turvy politics of love and marriage. And the music often sounds like a dry run for ideas he would put to more profound dramatic use in "The Marriage of Figaro" and "Don Giovanni."

Alas, there is almost certainly no such thing as an undiscovered Mozart opera. The life and works of few composers have been so thoroughly examined, down to the last detail. Moreover, the odds that anything Mozart wrote would get put away for posterity are dim; he was under such intense pressure to produce that his works commonly reached listeners virtually before the ink was dry.

But though it is not by Mozart, there is a gem of an opera that fits the description above of how a precursor to Mozart's masterpieces might sound. It is "Il Matrimonio Segreto" ("The Secret Marriage") by Domenico Cimarosa and a sparkling recording of it now arrives (on Deutsche Gammophon 2709 069, three records) under conductor Daniel Barenboim with five singers led by Dietrich Fisher-Dieskau, cast atypically and most delightfully in the central comic role.

"The Secret Marriage" is, however, no precursor of "Figaro." Its first performance came in Vienna in 1792, a bare two months after Mozart's death, and the work is more a successor to "Figaro" and "Don Giovanni." In it, Cimarosa, whose 50 or so comic operas utterly overshadowed Mozart's operas with the 18th-century public, shows that for all his personal success, he was not above learning from his contemporary. He obviously knew his Mozart, and he was not ashamed to let it show. He had a Mozartian libretto, and he set out to write a Mozartian opera.

It would be a mistake to push this parallel too far. Do not expect the symphonic dimensions and emotional range of "Figaro's" first-act finale. And do not expect the biting wit, intensity and utter individuality of "Don Giovanni's" own music in that opera.In general, Cimarosa seems less interested than Mozart in shades of meaning and subtleties of character.

But time and again, particularly in the second of its two acts, you keep hearing arias and ensembles that could be slipped into Mozart operas as lost parts just discovered, without their seeming, to the ear at least, out of place. One reason is Cimarosa's finesse in orchestration; as in much Mozart, light and flowing winds and violins carry the main line over an easy, steady bass - that is turned, typically, into a sustained pedal point for rhetorical emphasis.

Then there is Cimarosa's melodic and harmonic knack. To show the touch of sadness that underlies all this apparently good humor, there is the device of throwing a minor key accidental note into a bubbling melody. And when the character is genuinely distraught, as when each of the two leading ladies indignantly bemoans her misfortune at the prospect of losing the tenor each fancies (one is secretly married to him from the beginning), Cimarosa turns to the bravura leaps accompanied by descending chromatic scales that Mozart reserves for the passionate and wronged Donna Elvira in "Don Giovanni."

The story itself is a stylish romp, generated by a typically complicated sequence of misunderstandings and deceptions that conspire to expose the vices and vanities of all.

A difficulty in making an authorizative judgment on the performance is that there is little basis for comparison. This is the only performance in the catalog. But I fished up an old Toscanini version of the overture that had come from a 1943 broadcast, and am pleased to say that Barenboim, conducting the mellifluous English Chamber Orchestra, produces the more engaging, graceful version. In fact, the whole cast delicately balances the score's good-humored energy, on the one hand, with its lyric range of mood, on the other, into a mix as easy to swallow as champagne.

Fisher-Dieskau is the only big name in the cast, and as the nouveau riche father of the two sisters through whose marriages he seeks social status, he proves once again to be quite simply the most versatile singer of this generation; his clowning is infectious. There are no commanding voices among the other singers, but ensemble and style is what is needed here, and they have it. Julia Varady and Arleen Auger are the two daughters. Tenor Ryland Davies is the young man they both fancy and Alberto Rinaldi is the English count brought to marry one of them.

When the opera was premiered before Emperor Leopold II in Vienna, he was so ecstatic he ordered the entire work encored. One understands his enthusiasm. It is a shame that as Mozart's operas rose to their exalted position in the repertory, so fine a work became overshadowed. Barenboim's recording should help bring it out of the shadows.