Times were still tough in 1947/1948 so I would guess that they used leftover stock like many of the other makers. If not Boehler, then my next guess would be Poldi/Poldihutte, Vienna. But there were others:
Austrian Alpine Company
Kapfenberg(High Grade Steel Works) and Ternitz(I don't know who had control of these or if they were under Boehler)
Austrian Crucible Steel
Prague Iron Industry Company
Bohemian Coal and Steel Company - I don't know when or if the last 2 were absorbed or dissolved.
And of course there were other possiblities like Skoda, Rheinstahl and Gebruder Mannesmann, Max & Reinhard.
Either Max in 1887,
http://www.google.com/patents?id=Q8FxAAA...cad=0_0#PPA1,M1 , or Reinhard in 1890,
http://www.google.com/patents?id=DeFjAAA...cad=0_0#PPA2,M1 , patented a method of making seamless tubes and I can't say if Mannesmann made gun tubes or not.
Kind Regards,
Raimey
rse