636 is the highest serial number on a M.1924 marked Mannlicher - Schoenauer I have seen yet. To me, it was special order / custom rifle built on a left-over action and barrel. Steyr was always willing to concoct special rifles with non – catalogued features to the customer’s wishes.
The “High Velocity” monicker was not invented by Stoeger. It appears in the 1929 english language catalog of the Steyr – Werke AG. In contemporary Austrian German they called it “Hochrasanz – Repetierstutzen”, as did Burgsmueller, Kreiensen, in their about 1930 catalog. The also 1930 A.Frank, Hamburg, export catalog called it “Hochrasanz – Büchse” in German and “Repeating rifle for high velocity cartridges” in English. “Rasance” was originally a French artillery term for flatness of trajectory, but was used for bullet velocity later. So “Hochrasanz” was used for “High Velocity”. “M.1925” may come from a 1925 Steyr catalog which first mentioned the Hochrasanz – Stutzen. It was first offered in 7x64, 7.62x63 = .30-06 and 8x60 Magnum (S) only. By 1935 7x57, 9.3x62 and 10.75x68 were added to the caliber options. Apparently there were plenty of unloved (in Europe) M1924 .30-06 parts still available as this chambering was offered at a 10% discount at first.

The switchover from “Made in Austria” to “Made in Germany” and from the Austriaan to German proofmarks was not immediately on March 12, 1938, the day of the “Anschluss”. There were still guns and parts in stock with the Austria marking. And the proofhouses Vienna, Ferlach, Weipert and Prague continued to use their old stamps until spring 1940, when the new proof law and new proofmarks became mandatory for all “Grossdeutschland”.