Why solid 1970/71 15 x 6 steel rims are scarce

I want to pass along some information gathered while trying to locate 2 original type plain black steel rim for my
71 Dodge Monaco. Nothing earth shaking but possibly helpful to someone. My car came with painted steel 15
inch x 6 inch solid rims but the previous owner (I'm the 2nd) for some reason had replaced one rim with a 15 x 5 & 1/2 inch and the spare tire rim with a 15 x 6 & 1/2 inch, both of which weren't solid rims but had vent openings. Although the Monaco has front disc brakes...

Why solid 1970/71 15 x 6 steel rims are scarce
Author: eyemon