They were supposedly made for use as tickets to the "Goose Lake Music Festival" held near Jackson, Michigan in August of 1970.

According to the internet, 70,000 fans were expected, but over 200,000 showed up.

Apparently, there were nine variations of the "tickets" made:

The four I have show a solid hot stamp on both sides. I can't believe they have been around since the '70s. Any info? Thanks.