Hi Tom you are correct, they differ between model type not model spec. I know the 21 & 28 have different primary gears I think ones 24t and the other 25t but the parts list shows the r, se & sp of the model type share the same part number crank primary gear.
So basically if you are fitting a dry clutch to a mc28r you need the dry clutch from a 28sp/se or the clutch and crank primary gear from a mc21.