Brian,
If the letters are behind the 3 spot as you look
in from the 4 side on a polished dice, then
it is a security feature to make sure the
dice are from that casino. And not a pair that were
switched in by crooked players. Most
times they would put letters that were part of
the casinos name and some times it would be part
of the owners name. In this case it may be
neither. At any rate the same letters would
appear on the dice used that night. Most often
a stick of five dice would be used per shift.
Sometimes more dice would be needed, depending
if there was a lot of action on the table that shift.
The boxman and the pit critter would now which
letters to expect to see behind the dice.

I have dice that are not legal casino dice, although
they have casino logos. These are dice with double
numbers like two 2's two 4's and two 6 sixes!
I also have dice with loads behind some of the
spots!
Vegas was a wild and wooly town in the 40's, 50's
and 60's.

Peter