A bike that vintage (early to mid 70's, judging from the lack of braze-ons) will most likely have 531 tubing. 631 is, I think, post 80's tubing and part of their modern "air hardened" series.
Austro Daimler and Puch were pretty much one bike company when I encountered them in early 80's
I suspect that had very separate histories before that, but your bike seems to say that Puch was just Puch when your bike was made(?).....unless you see something saying "AD" on that frame somewhere.....
Chombi