It's the rated bhp, i.e. the nett horsepower measured on the dyno.
If you go back through the original data, then you find that bhp is at the end of a long chain of subtractions. IIRC it goes like this:
IHP>SHP>BHP
However the details are in
this book, my copy of which has vanished (presumably "borrowed") by somebody
.
It's the definitive work on the Merlin engine, and was originally published in 1941. You can actually build a pretty accurate model of the Merlin with the equations provided (and indeed, since they are general, you can also input data for the Griffon and get good results too).
Note that bhp doesn't include the exhaust thrust, which is very roughly 1/10th of the bhp in lbf.
For this reason ehp was invented in the turboprop era, though often the conversions used were somewhat approximate and primarily used for brochure purposes rather than performance calculation.