Though the words "Made in XXX" were prescribed by the British 1887 Merchandize Marks Act for all goods to be imported into the British empire, the conspicious "Made in Austria" marking appeared on the receiver ring of Mannlicher – Schoenauers in the 1920s only. IMHO this may be explained by the results of WW1. Up to WW1 the multinational Austro – Hungarian empire of the Habsburg Monarchy was officialy named " The Kingdoms and Lands Represented in the Imperial Council and the Lands of the Holy Hungarian Crown of St. Stephen". Obviously far too cumbersome to be marked on a receiver ring. Even "Made in Austro – Hungary" would have covered more space than availble. "Oesterreich = Austria" was the prescribed name for the country by the St.Germain "Peace Treaty" of 1920. Only from then on "Made in Austria" made sense.
But I doubt there was a clear cut date, as to models and serial numbers, when the phrase was added. The Steyr factory was not very consistent with their receiver markings. Already marked older receivers were used up alongside new marked ones, just like round and capped pistol gripped stocks. Sometimes even older marking stamps were used, maybe accidentially. I own a Mannlicher – Schoenauer rifle proofed in 1957. Clearly a post-1952 "Magnum" action without clip guide, but with a low bolt handle and additional side safety. The receiver ring is marked "Mod. 1950 / Kal.6,5x68"! Similar things happened at Mauser, Oberndorf too. I have a B Mauser, datable to 1930. The receiver is still marked with the old "Waffenfabrik Mauser" name, while the barrel rib has the new "Mauser Werke " inscription.