I guess you realize that, Mannlicher-Schoenauer's are an Austrian rifle made in the town of Steyr. I visited the factory there some years ago. Those marked "Made in Germany" were made after the Anschluss, meaning after Germany invaded Austria and annexed it in the late 1930's. Their advancing tanks were covered in flowers by the village locals, by the way, so most people welcomed them. So your rifle is still made in the same place, in Austria, just with different details stamped on the receiver as Germany considered Austria then to be a new state of Germany. Steyr were very busy making military rifles in WWII, and only a very few sporters were produced in this period, probably from stockpiled parts. I have only seen one Mannlicher marked this way.

There was no rifle production at Steyr after the war until about 1950 or so, when they were allowed to resume. The western allies were probably only too happy to have re-armed West-Germans and Austrians between them and hordes of soviets across the Iron Curtain by then. Much of the 1950+ production of Mannlicher sporters was for export, which might explain why the bore dimensions changed around then to .264", (but as I say, assume nothing without seeking proof if you want to use oversize bullets).