This is what is left of the tax stamp. It is distinguishable as a "1 pack" stamp.
Now that is interesting. There was a brief period of time where decks like this would have a seal stating that they're exempt from tax and to be sold only to military personnel. It looks like this one was sold through civilian channels, perhaps. Now I'm thinking it's even more likely that USPC did a MASSIVE run of these decks during World War II, leaving them enough leftover that it took a number of years to sell them all out. There wasn't much paper recycling taking place at the time and I'm sure it was easier and more cost-effective to sell them out rather than destroy them.
Either way, it's exactly the correct stamp for the period - the one-pack stamp was used from 1940 to 1965, when the tax was repealed.