Well, that does help to date it a little. TIG was available in ChCh from the 50's - there were a couple of specialist welders with the gear. If it had been MIG welded IMO that would have dated it as reasonably late work.
Gas of course has been around for years - but fusion welded gas is actually quite rare I'm told, most fabricators preferring bronze welding or refining it by using nickel bronze.
My personal preference based on 30 plus years of building roadrace motorcycle frames is bronze welding as I find distortion to be much less.
Carry on.