Redeemable item of yore

• Old-timey proof of purchase
• Proof of purchase for some contests
• Old-fashioned proof of purchase
• Proof of purchase, at times
• Cereal buyer's proof-of-purchase
• Cereal buyer's proof of purchase

Wikipedia Word definitions in Wikipedia
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 ...