TECH: Tin Primary Cover 101

The old, BLACK cover appears to be an ordinary XLCH tin primary cover.  1958 up XLCH.  You ever notice how many of these covers have the extra (screw hole) provision in the top center for a screw, but it's not drilled - and there's no boss on the XLCH case for the screw threads???  I believe KR and XLR and Iron XR have this boss, and it's tapped for an extra screw.  This cover is actually drilled for it (I presume it was run on a KR, XLR or Iron XR)  This cover also has that "extra clutch reinforcement dome plate" over the top of the smooth cover itself.  I'm not sure what year they stopped production of this earlier cover?  Later covers have the ribs, with no extra dome reinforcement.
Below:  KR, XLR, Iron XR, possible early Aluminum XR Cover
1. Top Center Hole Provision and Drilled
2. Heavy Clutch Dome reinforced
3. Possibly used on early XLCH ?
Below: (Same as above cover)
1. Center Provision NOT drilled
Below: Later XLCH Cover
1. No Center Provision
2. No extra plate over clutch dome.
3. When did they start using this cover ?________
I've noticed the small "front sprocket shaft nut dome" is slightly different on these late covers also . .  maybe just an entirely different stamping, or die stamp?  Anybody know anything about this?  Many old Harley parts all look the same . . . to the untrained eye.

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