Zach, thank you for the reply and links. The depth of info here never ceases to amaze me. I had no idea just how big the answer to my question would be!
I think I have the cream colored backs. I have two cards, the Mantle in first post and a Mays. Here's a scan of the backs, fwiw:
I suppose I'll send the Mantle to PSA for a grade. I don't think the Mays is worth it even if it is authentic. I imagine after that fiasco they are more on top of their game relative to these cards.
Thanks again!