Hi Raimey, as I have yet to see an official order as to when to replace the gauge numbers with mm ones, and all other sources seem to come from guesswork and contradict each other, I tend to date gauge numbers as "up to 1911" and mm numbers as "1912 to 1939". IMHO everithing else is like hairsplitting, though apparently the Zella-Mehlis proofhouse stated to use mm numbers about a year earlier than Suhl. But, from the introduction of the millimeter designations on, Z-M proofmarks can be distinguished from Suhl ones: For instance, a Mehlis marked 8x57R 360 falling block rifle is marked "7,7mm / 57 (case length) / 6.12 (June 1912)/ 51" and a 25-35 aka 6.5x52R Tell-type break open rifle "6,3mm / 52 / 3.36 / 534". I have seen those last numbers, here 51 and 534, but up to the 1200 range, only on rifles clearly traceable to Zella-Mehlis make and proof. My guess: these are sort of ledger numbers for the month of proof. So I think any rifle sporting such a number, even if marked and sold by a Suhl maker, was actually made and proofed in Z-M.