These are unused soda can bottoms that were intended to be used as seen in the pics below.

If you didn't like your soda in bottles you could still participate in the contest using cans!
Below is a rusty example of what these cans looked like, as well as a peek inside at the image visible there.
That's Don McDougall (The Guess Who) on one side and Ritchie Henman on the other side.

That's APRIL WINE's Jim Clench on the bottom - inside the can.

You'll just have to trust me on this.