The name on the side of the receiver means Austrian Gun Manufacturing (joint stock) Company and was used from 1869 to 1927 when it became Steyr Werke A.G., (meaning Steyr Works Ltd). Steyr is the town where these were made. So it will pre-date 1927. 1908 is the introduction date of the 8x56mm Mannlicher-Schoenauer cartridge. So this is the earliest date a M.1908 will have been made. Somewhere between 1908 and 1927 is the date of your rifle's manufacture.

There will be a proof mark on the barrel, hidden in the barrel channel until the stock is carefully taken off with good fitting screwdrivers. In fact, I'd recommend a good gunsmith do the job as he will be able to secure the rifle carefully before starting so it doesn't slip. The stock may well be stuck fairly tight and the king-screw may also be seized. A good gunsmith will know what to do next.

The proof number will be something like 1345.20 In that example, it would mean the 1345th proof done in the year 1920 when the rile was sent for proof. The small "w" (i.e. "NPw"), on the side of your rifle means it was proof tested in Wien, (i.e. Vienna).

I'm guessing from the serial number that your gun was made around 1920. By comparison, one with a serial number of 125XX was proofed in 1925. I don't know what the modified stock would add to the price, but it is not common.