If I recall, a Missa Solemnis was a setting of the entire Ordinary of the Mass, while many of the Mass settings of the time- Mozart's earliest ones, most of them, included- set, I think, just the Kyrie and the Gloria and were therefore Missae Brevae? (singular, anyway, Missa Brevis)- by contrast... Eric 02:10, 22 September 2010 (UTC)
The subtitle may be explained by this story if true. I believe you are right about the rest, yes. Eric 05:58, 22 September 2010 (UTC)