Hey Miguel @chorolekiam,
The information below can be found here: http://www.cypressfilms.com/bicycle/uspc_dates/uspc_dating_codes.html
For some time, there also was a stamp for tax-free playing cards. To the left is a stamp that was in use by the U.S. military c. 1946. Below are two examples of USPC playing card decks provided to the U.S. Government for military use in the mid-fifties. In the center is an enlargement of the tax-free stamp (all three images courtesy of Lee Asher from his informative article “The White Label” in Issue No. 13 of CARD CULTURE). Lee has since sent me some additional images of two League back Bicycle decks from his collection that also feature the white tax-free stamp, below.
Hope this helps 🙂