Take a ride on the stain train...
1957 Topps tough series Danny Kravitz has a small amount of non-problematic wax/gum residue of back. The 1964 Frank Malzone Stand-Up has a tiny bit of the obligatory wax/gum stain on its blank back, and both the 1965 Killebrew and 1966 high number Mahaffey look as clean as a freshly Windexed window. No matter how much they're tilted in the sunlight, I can't pick up even a hint of a 'stain.' Perhaps there's an unseen bit of residue that's only visible when the cards are actually in-hand and not in a slab?? Dunno...