The early GTA distributors have a larger diameter rotor mount shaft than some of the later Marelli GTA and GTA jr distributors. Caps do interchange. I have modified several Montreal rotors to work with my own GTA S119A Marelli distributors. The Montreal cap is not as robust as the old greenish GTA caps, but does work.
The value of having a real GTA cap on an S119A Marelli distributor on a real GTA engine, makes the Montreal cap better suited to a Montreal engine.
Yes, the original GTA caps are much more costly than Montreal caps, new or used.