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?
Music Fans Stack Exchange is a question and answer site for music historians, critics, and fans. It only takes a minute to sign up.Sign up to join this community
This is mainly a historical accident, arising from the fact that the pieces were published by different publishers. The piano sonata was published originally by Breitkopf & Härtel as Op. 81 in 1811. However, the publisher Simrock had also released the horn Sextet as Op. 81 in 1809 or 1810. It was only later that they were renamed Op. 81a and Op. 81b in order to avoid confusion.
Calling them No. 1 and No. 2 would indicate that the pieces are part of a set. In this case, there is no such connection between the them.
Source: Stewart Gordon, Beethoven's 32 Piano Sonatas: A Handbook for Performers.
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.