From what I understand, the "US" and "B" notations used on some T-mold orders were used to identify whether those chips were made were made by the US Playing Card Co., or the Burt Co. The transition happened around 1946.

I'm guessing that the color formulas used by the Burt Co. company were slightly different so that a Burt blue was slightly different than a USPC Co. blue. They probably didn't want to send out mismatching colors. There may have been another reason, but that is what I would guess.

I've wondered in the past if these color variations might be used to date some UFC chips.

You won't see those US/B notations on Harp mold orders, because I believe that only the USPC Co. produced chips on the Harp mold, and USPC stopped making them around 1946. If you look closely at the Taylor order cards you will notice that new orders on the Harp mold stop abruptly around 1946, and they are mainly using Harp mold chips during the late 1940s and 1950s for re-orders or add-ons for customers who had previously ordered the Harp mold chips.