Boxtop

A boxtop, in the context of being a proof of purchase, is understood to be the upper portion of a product box, detached, and mailed as part of a claim for a radio premium or other advertising offer. During the 1930s through 1950s, cereal boxtops were usually the most common proofs of purchase used to claim such premiums. (UPCs, which debuted in the 1960s, later served this purpose.)