More good data from you gents- how we learn. First off, any torch that uses oxygen and acetylene, no matter the pressure or tip size, will produce a neutral flame of 6000 degree F- about 2.25 times past the critical temperature of mild steel- critical temperature being that which causes a solid to become fluid in state- - molten steel in a crucible is at critical temperature.
Secondly- you won't get any stainless alloyed steel (mainly nickel and chromium- either 300 or 400 series no exception) to "Tin" for soldering with the heat and flux flow you can accomplish with mild steel and even cast iron-For the same reason you cannot cut stainless or cast iron with a conventional oxy-acetylene cutting torch-
Laser welding is the way to go- no preheat, no need to anneal or reharded to whatever Brinell or Rockwell range the parent metal had-
I may be wrong about the Parker process, but I know from Houchin's fine book on the Elsie's- he mentions a man who was foreman of the barrel brazing shop in Fulton for over 35 years- that the lugs were brazed, not soldered. Brazing requires a higher temperature than solder or silver soldering, the entire surfaces must be 'tinned" with flux for the bronze alloy filler to bond. BUT- both soldering and brazing do NOT reach the temperatures in the HAZ that welding- SMAW, MIG, TIG or Innershield MIG develop- where the parent metal and the filler (shielded from oxygen and in some cases- also hydrogen) reach the critical temperature and then cool down as the heat input moves with the travel of the weld deposit produced.
6150 Chrome Moly steel was developed about the time Parker added the 12.5 wedge to the barrel lug- that's my "SWAG" as to the nature of the alloy steel they specified.
Welding 101
Threaded Mode