There is an interesting parallel here with the autograph of Mozart's Quartet in D K.575, in which the cello part in bar 9 of the first movement is notated in the alto clef. Alan Tyson has commented on this strange phenomenon:
The presence of an alto clef on the cello staff at the ninth measure is indeed a puzzle; it does not seem to be accounted for merely by the reversal of the parts.
the editors have included two copies of the gamba parts, one in treble clef and one in
alto clef. The sonatas themselves are brief works of just two movements apiece, typically an opening AABB allegro movement followed by a rondeau or other character piece, and have a charming galant style with a hint of the early nineteenth century.
A few, however, have trio texture, and two of the concerts are recommended for two viols, the upper part being notated in
alto clef, although even here Couperin allows for the possibility of performance on other instruments.
The printed set of parts includes "Violino Primo," "Violino Secondo," "Terza Parte" (in
alto clef), "Viola" (in bass clef), and "Basso Continuo." The individual sonatas detail more specific instrumentation which includes "Violino" (1 and 2), "Alto," "Viola da Brazzo," and "Basso Continuo." "Violino" poses no problems.
Unfortunately, none of Lully's handwritten parts has survived, and they had to be created." Tafelmusik's acclaimed harpsichordist/librarian, Charlotte Nediger, translated Lully's score into modern treble and bass clefs (as opposed to Lully's soprano and
alto clefs), and wrote parts for the individual musicians.
Perhaps the biggest disparity in the parts lies with the trombone part, which contains bass, tenor, and
alto clefs. While many of the factors of this piece could make it accessible to an advanced high school or early college group, the clefs in the trombone part are a bit of an outlier.