Maybe far out, but it could be a reference to the B Minor Mass, at that time owned by Hans Georg N?geli from Z¨¹rich who was aiming at printing it (attempts from 1818, eventually happening in 1833). He is the one famously claiming the mass to be "das gr?sste musikalische Kunstwerk aller Zeiten und V?lker". Beethoven may have had a manuscript of only Kyrie and Gloria, explaining the reference to only 5 voices.
Best wishes, Niels brandt
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Niels Brandt