I'll try and address the '48-'53 covers. Below are 4 styles, two aluminum, A & B, and two cast iron, C & D. There may be more, But these are the only ones I've seen and removed so far.
Cover A with the lower shaft support came out in late '50 for F/M cars and trucks, as it has dual timing pointer locations for both the large wide-belt sheet metal
and smaller narrow-belt cast iron pulleys, and should only be used with the extended shaft aluminum distributor.
Cover B also has dual pointer locations and has
room for the extended shaft alloy distributor, but should only be used with iron body distributor because of the short upper support on the aluminum distributor, imo.
Covers C & D came on wide-belt large pulley engines because of the pointer locations, namely '48-'53 trucks and '49 Fords and Mercurys, and should be used with the iron body short distributors.
The second rear view picture, along with the third, shows the upper supports, and why the cast iron distributor with the stepped housing should only be used with covers B,C & D, and why the short body aluminum distributor can only be used with cover A.
The above is info is accurate to the best of my knowledge and hope it helps answer some questions.
