Just bringing this post up again - they were different weren't they? Wider hub w 0.5" more chain offset for the 5sp engine as I recall.
I remember when my 73 steel rear hub failed, we tried to put my mate's 75 5sp alloy hub in but there's no way it would fit in the 73 swingarm. Too wide. And even if it did fit, the chain would have been out of alignment.
How did you make them fit Mark? The pic above looks like the US sports alloy arm. Was it wider to cater for the 75 hub?