Don,
My usual go-to is "The Standard Directory of Proof Marks" by Gerhard Wirnsberger, published by Jolex. It covers all of the European Proof Houses with a very good German section and a very good summary of the British proofs.
My comment is slightly in error. The marking of the cartridge length became a requirement from 1925 and the marking of the service pressure was a requirement from 1955.
Interestingly, the German proof marks were accepted with no additional marking by the British proof authorities until that recognition was cancelled in 1946.
With the standardizing of proof testing by the CIP in the 1970's both Germany and Britain have mutually recognised proof standards now.
Mark
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