I found when putting an early cover on a later case that has the stator mount built in, the lower front screw boss interferes with the mount. When BSA added the mount they removed material from the inside of the screw boss to clear it.
The back of the cover would not rotate down to align with the case holes. A little trimming of the mount makes it fit.
The early cover also does not have the alignment dowel at the front. Only the chain tensioner post aligns the cover.