THAT'S what I call a christmas present Vince! I only had a Fazakerley casing handy and that didn't have the holes you mention present.
The D shaped holes are in fact ROUND holes partially covered by the ret spring cap locking ring. They were punched in as markers for the locking lugs of the return spring cap in the locking ring. Have a look....... The holes were designed into the manufacture so that in service, any crud or fouling that got caught in the locking ring would be cleared by the lugs on the return spring cap pushing it through the holes.
Not a lot of people know that!
Added a bit later: Just spoke to a friend and the holes you mention ARE D shaped on his (Sterling) but round on the Faz. But perform the same function. Also, various different locating holes too. Just different manufacturers being able to make best use of their facilities.