For a while in the mid 20th century it was popular for a business to attach self advertising stickers to coins that were given to customers as change. A cheap advertising gimmick. Several Nevada casinos did this. Silver Dollars mostly because a bigger & easier to read sticker could be used, but I've seen stickers on other coins. Occassionally I see a rubber stamp advertisement used on a current dollar bill.

The scans below are of a Sticker Dollar from San Francisco. I swiped the scans from an auction a while ago. Like a dummy I let this Dollar get away. If anyone has any info on this business I'd appreciate a post or an email.

BTW, my "Sticker Pfennig" measures about 16mm in diameter. Slightly smaller than an US dime. I do have an extra if anyone is interested.