The note is laid in MLM 44, Missa composita, et Serenissimo ac Eminentissimo Domino ... profundissima cum veneratione dedicata a Ludovico van Beethoven. Opus 123 ... Moguntiae, B. Schott filiorum, 1827.
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Beethoven mentions Missa solemnis is this translation of the note: "Here, my dear [friend] I make you a little gift, an English dictionary. With regard to the vocal pieces, I think we should let one of the singers, who is going to sing for us, first sing an air, then we would do two numbers of the Mass, but with German words. Look about you as to who could do this for us, it need not be exactly a masterpiece, if it only suits the music well. Yours, Beethoven."
The tenor Josef August Röckel sang as Florestan during Beethovens Opera "Fidelio" in 1806.