Around a year or two ago while searching Ebay I came across two separate lots with a handful of W575-1 blank back cards, both from the same seller. I ended up winning both lots. I had noticed on the scans that one of the cards (Whitted) in one of the lots had a back stamp, and that another card appeared to have one as well. When I received them, lo and behold, with the help of a magnifying glass, each card had this back stamp! On most of the cards it is only partial and barely visible, amazingly even less so than the McInnis shown below (have some fun finding the scant traces of stamping on the McInnis), but heck the stamp is still there on each card.
Here is a thread about them from 9 years ago, which makes the interesting connection between the Service Candy Co. stamps and the Henry Johnson stamps, based upon both companies having the same phone number.
https://net54baseball.com/showthread.php?t=170808
So the takeaway is to always have a magnifying glass handy when examining those blank backsides, because there might just be a trace of stamping there!
Brian