What's even more puzzling is that the Checklists are printed on white card stock while the other 120 cards are printed on grey card stock meaning the Checklists could not have been printed on the same sheet as the other cards. The Checklists therefore appear to have been an afterthought on Topps' part.
In fact offhand I can't think of any other Topps' set from 1957 or earlier that included a Checklist. The
Planes set might therefore be where Checklists were first introduced.
