Foolestroupe,
You've just explained to me the difference between 140 bass & 120 bass accordions, I think. It bugged me for years but I wasn't so motivated to actually get up and explore it.

Tootler,
Around the folk scene you'll commonly see Hohner Corona III button accordions. They're usually diatonic (like three-row melodeons) in A, D & G or G, C & F although I've seen them in almost every combination imaginable. Physically they're the same size as Hohner's smaller (40 bass?) piano or club accordions. Bob would probably be able to even quote you Hohner's model names. Occasionally you'd see a button accordion with a Stradella bass end. While most of these would have been Shand type 'British Chromatic Accordions (the B,C, C#) I've occasionally seen the Corona III with a Stradella bass end. Your sighting, though, is the first I've heard of the counterswap.

Bob,
You're right, of course about them being specialist but I got the impression that Dazbo might have (unexpectedly?) seen one. I've only ever seen one (40 years ago, in the hands of a Russian) and could recognise from a distance that he was playing it with dexterous use of the bass for a countermelody (tricky on a Stradella bass) but I have no recollection of whether the buttons have even the same physical appearance as the usual piano accordion