A common quirk of the old, 1893 – 1939, German proofmarks. The proofhouses did not mark the cartridge designation nor the groove or bullet diameter, but the bore or land diameter of the rifled barrel. This was measured using cylindrical plugs. The largest size plug hat passed the barrel was marked with the proofmarks. Until 1911-12 the plug diameter was expressed with a gauge number (number of lead balls to a British pound), then in millimeters in .1 mm increments. So a “9 mm” marked barrel may have been 9.099 mm indeed. To the bore/land diameter you have to add 2x the groove depth to get to the groove or bullet diameter. Often the maximum case length was marked too. The now familiar metric cartridge names, nominal caliber x case length in mm, were standardized by 1925 only. Only then the Mauser, Oberndorf factory started to mark their rifles visible with f.i. 9,3x62. Before, they marked just 9.3 mm. On the underside of an original Mauser factory barrel you will find more information: Land diameter- groove diameter – twist length and often the outmoded gauge number. But this did not apply to other gunmakers or proofhouses until the new 1940 proof law.