Bob,
You are quite right, after a fashion, and the story is a bit more complex than my simple sentence answer.
There were four main civilian proof houses in Germany: Suhl, Zella-Mehlis, Frankfurt, and Oberndorf. Krieghoff and Simson were located in Suhl and had their commercial reworks proofed there; Mauser was located in Oberndorf (the Oberndorf proof house may have actually been located in the Mauser facilities).
It is unknown which proof house serviced DWM in Berlin.
The first civil proof law in Germany, in 1891, specified (among other things) the c/B,U,G suite of proofs. These proofs were based on black powder. When smokeless (nitro) powder became common, it was realized that the 1891 proof requirements were inadequate to the new technology.
In 1910 the proof houses of Suhl and Zella-Mehlis developed administrative regulations based around a "semi-smokeless" nitro powder which included a new marking protocol, the crown-over-N.
In 1911 or 1912 (there is conflicting information) the proof houses of Suhl, Zella-Mehlis, and Frankfurt began proofing to this new standard, with the new mark.
The Oberndorf proof house did not "sign on" to the new system, and continued to use the 1891 c/BUG proofing method, which is why your 1940 training rifle example still has c/BUG proofs.
Of course, Germany established an entirely new proof law in 1939 which became affective in 1940, which introduced the eagle/letter proof mark.
--Dwight
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