Beethoven has an Opus 81a (Piano Sonata No. 26) and Opus 81b (Sextet for Horns and String Quartet).
Why is this Opus divided/labeled this way, instead of split into Numbers as opus cataloguing usually dictates?
My best guess is, that numbers are used more frequently, when grouping very similar works, for Beethoven the violon sonatas, the string quartets op. 18 and op. 10 piano sonatas.
Here there is not much common beneath the time of creation (1810).
As the piano arrangement of the violin concerto shows, directly derived works also receive a letter, as op. 61a opposed to op. 61.