Hi Louis,
of course I have seen your rifle on the forum. But as I can not positively date it, I refrained from posting myself. As you asked me directly now, let's give it a try.
The rifle was certainly proofed in Austria too, as required by law. The original proofmarks on the barrel reinforce are covered by the soldered on front scope base sleeve. These "factory" Vienna style mounts were not made by the Steyr factory, but mounting was farmed out to several independent, outworking gunsmithes in the Steyr area.
The only chance for precise dating would be a proofhouse ledger number + year, most often found under the barrel in about an 8 o clock position. Without such a tiny number there, it is mostly guesswork. The Steyr serial number on the right side of the receiver ring is of no help as the Steyr factory started each model over again with "1". Production records are lost.
The front sight with base is the standard type, merely soldered on about 1" back from the muzzle. A feature quite often seen on detachable stock "English models" made for the British market.
The Voigtlaender "Skopar B" originally on the rifle was offered from about 1905 to the mid-1920s.
According to my references the wording NITRO PROOVED was introduced in 1916.
On foreign made guns the London Nitro proof mark "Cleaver over N" was applied with a circled N from 1925 on.
So I dare to guess: Your Mannlicher – Schoenauer M 1908 was most likely made immediately after the end of WW1, maybe about 1920.
Waidmannsheil!
Axel