I am really not sure when the oxy-acetylene torches came out, but that process preceeded electric arc welding with the flux coat stick electrodes (SMAW)- But acetylene and carbide with water were used for miner's head lamps before 1900 I am sure-

My guess- the lug and assembled barrels were pre-heated by forge to allow the flux to flow evenly (tinning) and then the brazing metal added, clamped into a fixture (possibly like a sand mold cope and drag, not sure) and allowed to cool to ambient temperature- but that is only speculation.