The "BH" stands for "Brush & Holder" This was meant to be either a travel brush, or one that you could stand on the counter in its matching holder.
Thanks for the info! That explains the handle shape, as well as the alpha code. I have an E-R Badgerlon, where the BN code says 'badger/nylon' to me.
Given that the code is not about the knot, I am curious still about the knot hairs. The inner hair is very soft and pliant, nothing like the usual E-R banded boar knots. The outer hair has a little backbone, but there is very little of that hair.