The Mass K.262 (246a) is known as Missa longa. Leopold Mozart designated it so next to the incipit on the cover of the autograph collection of five masses in C major by his son. Mozart literature gives the year of origin as 1776, without mentioning the speculative nature of this claim.

The monumentality of the Missa longa suggests that the work was written for a particularly solemn occasion. When scanning for outstanding ecclesiastical events in the 1770’s, one cannot ignore the consecration ...