C2cigars has the rigt answer.
Typically the Mail-in "cards" were part of a sheet of the full set of checklists on very thin paper stock not typical thicker cardboard stock the regular cards were made of. Naturally most people back in the day separated then into individual cards. I never did the mail ins and I don't think I have gotten any in recent years but have heard plenty of stories and reports of them. I think many kids who did do the mail-in thing were disappointed when they actually came in one large sheet and thinner card stock.
I collect: Baseball, Football, Hockey, Mostly Vintage pre1980, My Homie teams - Washington/Baltimore Teams Senators (Twins, Rangers), Expos/Nationals, Redskins, Capitals, Bullets/Wizards - HOFers - Non-sport (mostly TV shows and movies).
My Trade List is very much a work in progress
CaptKirk42s Trading Card Blog Curly W Cards Strive For '65 YouTube klandersen42