book "Guide to Collecting Casino Dice" Las Vegas edition:

"Another innovation are serial numbers... Serial numbers first appeared in the 1940's, but were not used widely until the 1970's. Dice from the 1980's and 1990's will have serial numbers almost without exception.

Key letters are another innovation. A key letter is a letter underneath the spot on the dice. It is used as an extra precaution against cheats who might switch dice into a game. These key letters have been found under the one, three, and five spots. These are the only spots that are in the center of the cube and are easily seen by the pit boss when he inspects a die that has gone off the table. The key letter is applied after the holes have been drilled and before the spots are painted. The key letter was offered by manufacturers as early as the late 1940's, but has not been found in casino dice before 1970 with very few exceptions."

That doesn't exactly answer your question, but it's interesting anyway. We have some dice with key letters, but not a lot of them. Then again, some of our dice are from the 1940's, 1950's and 1960's...