1960 Fleer Football - what's up with all the "proofs?"

[G1911]

I was surprised to pick up this Jack Kemp that is misaligned, has the wrong reverse (of 51 Billy Kinard), and missed some of the color ink runs for less than half of what the proper Kemp card sells for. Doing some research, I see quite a few "proofs" are out there, particularly the yellow inks, with multiple cards of one player. I've never paid much attention to the Fleer's before, but I've never seen this many pre-production or partial production cards before. For example, there are 2 yellow Kemps and 2 blue Kemps like this one (though they have the correct back, unlike mine) listed on eBay right now. It seems like almost all of these have the correct backs, if off-centered on the reverse more than the front

Did these all come from one find? Does anyone know the story here? I always like blank backs, proofs, misprints and other oddities. I'd love to see more in your collections!