SEEKING CAL 500 SLOTTED MAG CENTER CAP SOLUTION
I have about 15 or more of those wheels but only a few centers and none of them are bolt on. Instead, they press into the centerholes and are held captive by plastic hooks on the tabs that press thru the centerholes.
If I remember right (haven't seen then in decades) they are in good physical shape but not so good aesthetically.
If yur interested in that kind of thing, I'll go out to the shop after supper and see if I can find them.