RS 3 spoke steering wheel differences?

[Mr Day]

I recently bought an rs wheel just for the centre cap for the rs wheel i already have on my car,today i took mine off to compare them and there different,the one on my car has no markings or stamping on it and the other has the ford logo and number stamped on the back,the threaded nut bit on the inside is also slightly different,does anyone know if certain ones came on certain cars or was it just a case of ford chucking anythin on any model?Also were the mk1,2 and capri steering wheels all different or do they all fit all cars?The thread bit measures about 18mm!I'd be greatful for any help!