Hey look at the time (OK that makes for my excuse for the following theory)

Lets say the casino used L mold chips, with their name, in various colors, for the games. But running low and getting some new chips, they decide to make the old chips into 50c (or anything else they wanted) by placing a value on them.

Many poker games are played with unmarked chips, where it's possible for a game to be 25c, $1, $5 one night and $1, $5, $25 another using the same chips.

So the old Black and Purple chips were stamped to make some chips with marked 50c values, to add to the new set? Re-cycled chips...

Complete, WAG, but after seeing rubber stamped roulette chips from Harolds Club, who knows?