Pro Cantione Antiqua
Mark Brown, director
Scores at IMSLP Note: I only looked at a couple of the many scores available and I wasn't sure of their accuracy, so you can judge. The one with the most stars the manuscript listed as "Anton Friedrich Justus Thibaut (1772-1840)? former owner" looks interesting but I doubt anyone could use it as a performance score. It uses c clefs.
Note, this is the famous one, the one that the teenage Mozart heard once at a Wednesday Vespers service at the Vatican, wrote out the score from memory (he had to go back to hear it again and he needed to make a few corrections on Friday) winning him fame for his incredibly good ear training and mastery of harmony and counterpoint and earned him him being appointed chivalric orders by the Pope. Never let anyone tell you that the Mozarts weren't masters of PR. One online source says that the English music commentator Charles Burney got a copy of the score from Mozart so I would imagine the IMSLP scores published by him would reflect that line of transmission.