Car Code for Brakes

I'm looking at the '73 Shop Manual under disc brakes. The table listing the rotor thickness separates by car line. What is "R-W' and "B-J' in the table? Typically "W" is Challenger and "J" is Coronet/Charger. "R" is Windsor, "B" is Hamtramck. But the combination of letters is throwing me. Would they have used a different rotor spec for a disc brake on a Charger at the Hamtramck plant than one built in any other plant????:realcrazy:

Look in the begining of the FSM for the codes and abbreviations, etc
I did that. That's how I know that w is for challenger and j is for coronet/charger. But there is nothing that combines w and j with R and B.