It is akin to what people did with bookplates in books; just IDs it as from your collection. Heck, I knew some kids who used to write their names on their cards. It isn't like they were worth anything back in the day. I love this card:
To me it is a blank-backed scrap from an OPC sheet but to Carmine 40 years ago it was a valued part of his childhood collection. Or this folk-art card:
Someone took a 1940s Coney Island card of Sullivan and created a custom mount for it. It is part of the charm of collecting old items to see how the wear they accumulated reflects how they were owned and used over the years. Someone went to a lot of effort to make this card; they actually made a whole set like this.
