The mk3 will only fit with mk3 switchs as it is flat where the four mounting screws are. In the same area the earlier master cylinder has a half circular channel where it matches the same in the switch to form the clamp that holds it on handlebar. Also mk3 has a different thread for fluid cap as it uses a plastic cap not the earlier alloy type. In all other respects they are the same but you will need correct cap an switch for whichever type you use.
They should both have same mirror mounting threaded hole in the same place. There are some rare early ones with no hole at all for mirror these where said to be for countries that drive on the wrong side of the road.