"I believe that ALL poker chips made from the turn of the century to the late 1930s were made by USPC. From the late 1930s to the mid 1940s both USPC and Burt made chips. From the mid 1940s to the 1970s Burt was the sole maker of casino/poker chips."

I think you are close to what I would have said.
TR King also made clay chips, but % would have been very small when compared to USPC & Burt. I am not sure if they made Poker Chips.