I'm not sure that there IS an obvious clear reason for using a different return spring cup unless the diameter of the inner return spring on the Mk6 is a different diameter. There are MANY variables of these semi guns......... Some changes were made simply in order to satisfy the local authorities, such as the BATF that the internal parts weren't interchangeable.

Is the 'longer' return spring cup longer externally AND internally or just externally but spaced internally? Why don't you just try the standard L2A3 cup, return spring and see if it operates?

As I said earlier, there were many variables in these Mk6-on guns for several reasons so the answer is that as a general rule, the parts are interchangeable but on some guns, certain features, such as the mechanical safety plate were omitted and on others the trigger mechanism blocking plate (that prevents the insertion of a rock and roll trigger assembly) differed