Heinz,
Correct, actually it's an O-umlaut, so Boehler is also correct. Stahl is indeed 'steel'.
The advert can be exactly dated to 1913 as it comes from a 1913 publication.
It's interesting as Boehler write in the advert that they supply barrels for machine guns and pistols to most army contractors. Also note that they have a representative in Luttich (or Liege / Luik) in Belgium, home of FN and Bayard/Pieper, which later became part of FN. Another interesting name is the Klett firm, also known to have done P08 reworks.
Boehler Stahl was a household name for barrel steel in early 20th century Germany.
A rough translation of the ad:
Boehler Steel
For every rifle the main value is determined by completely flawless barrel material,
because safety of the shooter comes first and that can only be guaranteed by first-class
material. We do not only deliver barrel steel for machine guns and pistols to most army industries
but also manufacture the following rifle brands for hunting rifels:
Boehler antinit, rustproof
Boehler special steel
Boehler-blitz steel
As trials at the weapons technical test institute 'Neumannswalde' have shown,
our references are first-class.
Boehler antinit, rustproof stands out because of it's excellent anti-rust qualities, but
especially because of it's extreme solidity.
Brothers BOehler and Co. AG
Berlin - Vienna
Dealers: Carl Bittiner, Liege, Belgium - Gustav Bitnner, Weipert, Bohemia
M. Ogris, Ferlach, Carinthia - F.A. Klett, Suhl, Thuringia