'does that layout bear any resemblance to any of the big square German boxes?'

Not any that I've seen, Richard, though I don't remember ever having a go on an American polka box. The chemnitzers I've got from Argentina and Germany seem to be anglos centrally with lots of extras round the outsides like my Wheatstone was when I got it.

'I would never manage to work out a good arrangement, and it's better really to leave it as built.' I would have to agree in principal. I was very lucky with mine though I had to pack some of the slots when I moved the reeds around and on the bass side there are still some restrictions in D and A in that I don't have the full 5 keys. I'm learning a tune in A for our next album and it briefly goes below what I have on the box. Might have to dig out my old DA box.

The logical conversion on this one would be DAG but as that is going down the scale you would need bigger slots, but not by lots. Would new reed pans be prohibitive? There are lots of quality reeds there to play with.