Found the answer to the large and small ID roller cams... Picked up another brake from Roly the other day and one arm has the small ID but the other arm is the large ID version but it came with a sleeve that fits inside the brass tube which effectively reduces the ID so that it can fit on a U-brake/roller cam brake pivot. Why they did this I've no idea but the side profiles are different (see picture).
Anyway after all that I'm still missing bits to make a whole set, so if you have a broken one let me know as I need to cannibalise it!
I think the idea of the sleeve is that when fitted, the brass bush is only in contact with the sleeve and not the studs on the frame. That way. the studs on the frame never have any wear on them. Same idea is used on various dia-compe cantis.
I guess that's the best theory so far but you would have thought that this variant would have come about in time i.e. though continuous improvement but both were made in July ’86 (CG casting), so they were available at the same time, which is puzzling.