This is a question I can answer.
It looked like this:
Sort of an almost square-looking rectangle in the post-1952 packs.
This is from a pack of 55 Bowmans I bought for $100 back in 1990 or so. They came from a box that had gotten a little bit of water damage. I bought three packs and opened each one, and in all three, the gum had sort of rotted and stuck to the card.
I still have this card, and I haven't removed the gum because its awesome.
-Al