31 is the year of proof, 1931. From the late 1920s on the proofhouse ledger number was not marked on Steyr made rifles any more. Apparently the Vienna proofhouse simply listed them by serial number from then on.
I don't have much faith in the style of receiver inscription for dating. Sometimes a workman simply grabbed an old, outdated stamp and applied it. My own 1957 proof dated 6.5x68 M-S rifle is marked "MODELL 1950" on the receiver ring, though it is obviously a post-1952 "Magnum" action without clip guides, with an additional side safety and low bolt handle.