The crown over N is the standard German Nitro proof, nothing to do with the Nirosta. I don't think I've seen pre-war guns made completely from stainless steel either, the Nirosta was an option for the barrel in some Sauer guns. A lot of "stainless" looking guns from that time are actually made with aluminum frames.
Found this little tibbit googling:
Stendawerke Waffenfabrik GmbH of Suhl, Germany took over production of the Beholla pistol from
Becker & Hollander shortly after the end of the First World War and after some design changes to simplify
field stripping, manufactured Stenda pistols until about 1926. Stenda pistols with serial numbers into the 70000
range have been observed but it is thought that the numbering continued from the Beholla period and so
probably only about 25,000 pistols were manufactured by Stenda.
That explains the BH monogram on your gun.
F 135 - the right choice