A true oddity. There is a 1969 Topps Crazy TV test set with fronts that are indentical to these, but the backs have knock knock jokes. The blank backed versions came in packs of Tons O Gum, but are not to be confused with 1969 Crazy TV Knock Knocks also available in Tons O Gum which were just knock knock jokes (the same ones that would have appeared on the back of the Crazy TV test run) with a blank reverse. Fir the record, there was also a card series called Tons O Laffs that was available in packs of Tons O Gum, which seems to make more sense thematically than either of these. This cardHas multiple spots of gum residue on the front, is off center top/bottom and miscut left/right