You are right, .30 is the proof date. Since about 1928 the Steyr factory department of the Vienna proofhouse did not stamp the proofhouse ledger number. Perhaps they registered the M-Sch rifles by their serial numbers then. The M1910 serial number should be on the right side of the receiver ring, above the woodline. The Steyr factory used seperate serial number ranges for each of their Mannlicher-Schoenauer models, so a comparatively low-numbered HighVelocity rifle, M 1925, may be much later than a higher-numbered M1903. FI my own M 1924 in 30-06 has the factory serial number 299, while my slightly earlier M 1910 is # 11400.