Been having some fun researching the archive to find out what the BR stamp behind the strainer means. Some say it stands for brass; others say B stands for brass and BZ stands for bronze. .
I think they used all three notations to identify shell material over the course of time. First they used a "B" stamp by the muffler to denote a brass shell and then later, when they produced both brass and bronze shells, they switched to the "BZ" and "BR" notations to differentiate between the two B metals. A brass Monroe shell would have BR since they also were making bronze shells at that time.
The early bronze shells didn't have a stamp, apparently since they had dropped brass and didn't plate any bronze shells so it was obvious that they were bronze.