What is this clef in Vivaldi RV281? This is the only source of the concerto is available ( at least without buying it, which is the manuscript). The piece did have a few weird clefs like an octave bass clef once for one of the violin sections, but nothing too unusual. His handwriting is pretty hard to read sometimes, and I am just plain confused about what it is. The section that played the stave was the viola section originally, and it switched to this clef later on. I remember seeing something like it before, but I forgot what it was, looks kind of like the French 18th-century clefs, but neither that nor alto clef sounded right or true at least to the recording I am using, which is essentially the same all the other ones, just a semitone lower (for baroque-interpretation, although I don't really care), here is the section in the youtube video I'm using and below it the picture of the clef in question,.
It might be drawn a bit scribbly but it certainly looks like a K-clef, (a stylistic variant of the C-clef). In particular it's an alto clef (which makes sense for viola).
So those first few notes in the image are at the pitch of E above middle C. (With the two treble G-clefs directly above, the chord made (E, G, B) is an E minor chord in close position.