Brien;
I have been collecting casino playing cards since 1975 and believe me a lot of my older decks are not cancelled. Some older cards in my collection that were cancelled, some by drilling or punch, some by using a magic marker on the side and some by stamping (with ink) on the front or back. The shaved corner cancel started appearing in the late 1980's and is a very common cancel right now.

The casino I work at is currently shreading their playing cards until they can get a new cancellation machine. The old company used to send them off property to have them cancelled. What a waste. I don't have any!

Maybe some of the cards that you purchased from the Gamblers General Store were of a design that was refused by the casino for one reason or another and then sold to them as surplus. On the other hand I could be wrong, a few years ago I remember purchasing Binion's Horseshoe (LV) cards (the same back design they have been using for nearly 35 years) from a little gift shop somewhere on Fremont street and they were not cancelled.