It is a "security key" and is one of the feature that can be added when ordering casino dice. It allows the pit boss to quickly see that the dice have not been switched. It is usualy on the back of the 1 or the 3 as is easy to see by looking through the die but can be on any spot. I have some with the security key under a corner spot on the 5.

Unknown Dice of the day:
These dice are celulose nitrate. The red are starting to crystalize at the
corners. The white ones are little yellowed but solid with no deteioration.
The spots on these dice are drilled and painted rather than filled in with
plastic like newer dice. There are no serial numbers or cancelation marks,
only the monogram ("C" or "G"?) on the 6 side.