Here is a brochure from Brenneke in the early 20th century. You'll note that there was a very costly option "Bohler Antonit Steel" barrels. Bohler Antonit is a high cost steel that was used on German best-guns including the modern era Beretta SO series shotguns and their double rifle counterparts. They called it "non-rusting" but it is not stainless steel to my understanding. However, its composition results in a different reaction to bluing than other steels. When I've seen it on vintage guns it seems there is a preference for a plum brown finish on Bohler Antonit barrels, probably because it matches Boss' London styling and also because it permits a barrel contrast rather than trying to get an identical match against two totally different steel alloys. (action versus barrel steel)
Keep in mind, this was a 5.15 Great Britain Pounds option in 1915 just for Bohler Antonit barrel upgrade! In today's money that's about 650 pounds or $850 dollars.
***Please keep in mind Brenneke did not make rifles, they just sold rifles for export and wholesaled to the trade. This gun is a custom rifle and has Brenneke inscribed on the barrel but it is hard to determine if it was wholesaled by Brenneke. It has many of the features of the extremely valuable ESHA model 21 and appears that it was a model 21 deluxe that had a scope installed necessitating the change of the ESHA cocking piece to a low mount and replacing the esha proprietary greener safety with a standard greener safety to get it to be compatible with the scope system. The retailer is also on the barrel and the double trigger system (a sealed unit like a timney) so this addition of a scope and replacement of the greener safety either happened before delivery or it was brought back to the original retailer for this work.
View attachment 366554