I just finished updating my 1959 and 1960 sets to include cards 199-286 (1959) and cards 375- 440( 1960) in both gray and white back. Have not decided whether to do the same for 1953 ( 86-165 ), 1956 ( 1-180)....let alone 1952 (131-190)and 1954 ( 1-50)
I guess if SCD did not list the MBs as a separate set, but instead treated them like the above back differences, or the 62 greenie fronts, they would list the Cox and Brinkman cards as variations, like they do now, but drop the separate listing for the set and just note, like they do for the above set, that certain cards in the set can be found with different backs
But if they did that, they would have to contend with the different backs in the latter series. That would get pretty complicated. If the MBs are added to a master set list for 1968, would that mean you would have to list the alternate latter series backs as well
I think the 1991 Topps sets can be finished with 3 or 4 different backs. Including the Desert Shield set it would seem to have more variant cards than any other Topps set