I don't see very many SSPC cards on Listia, but recently there were quite a few up for grabs.
I managed to snag three.
The front of the cards are 3 9/16/' X 4 1/4" and are all posed shots.
These are from a twenty-four card set and with the backs you can put together a puzzle of Nolan Ryan and Catfish Hunter.
I had no idea what these cards were when I bid for them because the description only listed them as 1975 puzzle cards. The only mention of SSPC is on the back of the cards (and I didn't get the piece or pieces that featured that).
I did run into a little glitch receiving them, though. I got an email that they were sent on the 9th, but when I received them much later, they weren't postmarked until the 20th. Not a problem, but I did email the seller on the 22nd for a status update but never got a response. I was just about to request a refund when I received them.