As I recall the 1939 rules would not allow the 0.318" chambering
No, the 1940 proof law merely prescribed the clear-cut differences between bullet diameters and the designations 8x57I and 8x57IS, extended to other 8mm cartridges like 8x60S, 8x68S. It also prescribed minimum dimensions for both the ordinary, I or Normal, .318" diameter and the S or Magnum .323" diameter. It also prescribed all rifles to be marked clearly with the name of the cartridge, fi 8x57IS instead of the formerly used bore diameter 7.9 and case length 57. Cartridges were only allowed to be sold as complete,sealed 10 shot boxes. Boxes containing S bullets had to have a black stripe behind the cartridge designation and a warning "for use in S barrels only". Individual S cartridges besides the headstamp were to be marked further with a black primer and a stippled line around the bullet. Until very recently every candidate for the Jägerprüfung = mandatory hunter's examination had to know these rules and markings. The I .318" diameter was regarded a the "civilian", while the S .323" was more the "military", but both cartridges were loaded to the same ballistics in 8x57. Only some time after WW2, when german hunters were allowed to rearm, the I diameter went out of use, because most used sporterized K98s, remains of the Wehrmacht pulled out of the bushes or rusted out prewar rifles rebarreled with plentyful 8x57IS military barrel blanks.
