it's ABP - shorthand for American Brilliant Period.

A style of cutting that apparently appeared simultaneously in Europe and the States........Ken says from about mid 1870's to about end of the first war - typified by masses of deep cutting and a minimum of surface area remaining plain. Usually has the pin wheel on show somewhere, and lots of deep mitres. Have a look at Charles Hajdamach's first book, chapter 14.
This one looks a bit off the wall for RB to me, but then am sure you know more about that factory than me.
Sorry this isn't much help.