Tyrus;

The stamp/seal you found on the card decks
is a tax stamp indicating payment of a tax
on playing cards that was in place from the
late 18XX's to 1965. The tax stamps were
bought by the card companies from the US
gov. and applied to their finished decks
by the card companies indicating tax payment.
The initials of the co. were applied to the
tax stamps as a precancellation to indicate
who used the stamp. After the tax ended
card companies continued to use a seal on the
decks to asure to deck users that the deck
is fresh and not tampered with.

Jim