N15CS primary cover

Hello I am looking for a outer primary cover for N15CS. So ….. when and why as the smaller outer bulge on the larger front bulge which covers the alternator introduced. I have seen a few and all seem to have extra clearance for the alternator nut on the inside of the cover but some have the extra bulge on the outside. Are they interchangeable. Cast on my inner is number 025017L. Thanks richard