Finally, in un-clipped my pencil, reduced all the (for the lack of a better term) misspelled pitches to their most common enharmonic names, at which point (yeah...I'm dumb) I finally realized that it's nothing more than an awkwardly-spelled (in order to stay within the key of four sharps - though naturals could have been utilized, but - then again - I believe Bordogni was striving to teach his vocal students to read) C minor scale.
I'm usually pretty good at recognizing such things on sight, but - well, obviously - not this time.
Being an old guitar player, I "get" C-minor, though...(but maybe not-so-much B-sharp-minor-on-a-B-flat-tuba-down-reading-in-string-bass-clef).
fwiw, this is Rochut #91 (whereas several of the subsequent etudes - even #120 - are a good bit easier - at least, for me...played with this instrument).
Something else that I have found necessary with a gigantic tuba with a gigantic bore size (21.2mm) and rotary valves is that - unlike F tuba, E-flat tuba, euphonium, a smaller B-flat tuba, C tubas, and piston tubas with 3/4-inch bore (etc.) - I'm occasionally needing to sacrifice the best in-tune valve combination (ie. the "normal" ones - those found in the fronts of beginner band method books, etc.) for more easily-controlled legatos (ie. slurs) with not-the-best valve combinations...It's a bit reminiscent of the dancing hippos in the Disney classic, "Fantasia".
