In 1965 Robert Münster, the Mozart scholar, determined that in a manuscript copy containing 13 settings of the "Tantum ergo" by Johann Zach [1669–1773], the Director of Music at the Mainz Cathedral, the final "Tantum ergo" in B flat major was for the most part identical to KV 142. Zach’s work is 43 measures long instead of the 63 measures of KV 142, although the Amen coda is missing. For some unknown reason, Mozart made a few changes to Zach’s composition and through the addition of the coda ext...