Canada in fact gave quite a lot of M1917 rifles both to Norwayand Denmark
after WWII.
And to the question how and when it came to Austria: after WWII Austria was split into four occupation zones: French
, British
, US and Russian
. Nearly all of those four left lots of their weapons with the new Austrian "B-Gendarmerie", the first Army of Austria post WWII. A friend of mine did his Military service in Austria with a Russian M44 Carbine - and the police in Kärnten (English "Carinthia") was equipped with Enfield No. 1 Mk.III rifles (remember the "ÖSTERR. GENDARMERIE" marked Enfields!). Therefore the origin of the two M1917 rifles which were converted might not be easy to answer, but for sure lies within the supply and leftover items post WWII.
Another funny mention would be a different rifle of the Austrian Army: they were also supplied Springfield M1922M1I, M2 and M1922M2 rifles (and also Remington 513T rifles). They did not knew what to do with those, therefore they gave it to Biathletes to have something to train with. Therefore they turn up from time to time in Austria.