Here's the pic of the guts of my "mystery" digitised 21900, Peter, but as per previous discussions it is of unknown provenance..... still don't know if it's a factory, dealer or personal conversion, and I have no idea whether the internals are anything like the production 23900 versions. As Gizzy said, it's got an old 55020 in it which is due for direct replacement with a Massoth L, plus an added Massoth S for sound. As it stands, both coupler motors work together via a little black box (presumably a relay) in the centre of the board, controlled by F1 (one push opens, next push closes).
Don't know if this is any help at all?
Jon.