Made In Austria was added to receivers of new production MS beginning sometime in 1924.

The Oesterr Waffenfabr Ges-Steyr (OWGS) stamping was changed to Steyr Werke AG during 1926.

Your proof marks look incomplete to me if pre WW2 Austrian. I suspect the receiver and barrel were proofed very early in the Anschluss period when the system changed as Hermann Goering was handed the management of Steyr Werke as Austria became Nazified. Pre Anschluss Vienna Proofs look like this:

Shown above is an M1903 (6.5), the 411th proofed at Vienna during 1908.

Yours does have (pre Anschluss) Proof House and 'Nitro' marks, yet seems to lack the Austrian numbering system.

I see no indication of the barrel not being original to receiver.

Shown above is the 'Anschluss Eagle'used during 1938 - 45.

This, however, is the Anschluss era side rail stamping:

Considering the above my initial suspicions are that your rifle was assembled of OWGS produced (pre 1926) parts in 1938 and presented for proof early in the Anschluss period. From examples I have seen on the 'net over the years this would not be as unusual as it may seem.