Are you positive on the lettering or going off of PSA's site? Their lettering is off as they list Mantle as G.
I do believe these to be a Wheaties premium given the similarity between these and the 52 Wheaties.
This set is strange. Looking at the population reports for PSA, it looks like there was a stack of these stored away somewhere. There is an abundance of mid-grade cards, which is right where I would expect a card to fall that was stuck in box untouched for 60+ years. My two are posted below. One clearly shows its age/wear. I tend to believe the test issue theory, particularly given the distribution. It would not surprise me if they released a handful of these and then decided to nix the idea.
