Did you check to see if the print quality matches the print quality of a card? If so, if it isn't a 2nd generation print, then you may have something there. The company's designers did make paper prints for quality review during the design process. You might have a cut from one of them. Here is an example from the 1948 Champions set:
I also picked up this Mays that we couldn't figure out either. It was labeled a 1952 proof but was printed on thin card stock different than the regular issue stock:
