Look closely, it may be a B. The numerals are the same as the rails so it has to be either a B or a mistake. I can understand mistakes on any digit except for the last one, the Acacia Ridge plant should never have got an S mixed with a B.

BHJ10949S is also a 1/75 or 2/75 number at the latest. BHJ10949B is a 3/75 number. I reckon if you pulled the plate off and cleaned it it'll be a B.