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Bulldog:

I don't have that book, but many other books say the same thing. They're technically wrong. Crown over BP (which stands for Birmingham Proof), introduced in 1904, is the Definitive Proof Mark of the Birmingham Proof House, and simply signifies passage of final proof and is not specific to propellant type. Prior to the 1954 Rules of Proof, there was no such thing as a "Black Powder" proof mark. The reason for the confusion is simple. Under all British proof rules prior to 1955, ALL proof was for Black Powder ONLY - UNLESS the Definitive Proof Mark was accompanied by marks indicating that SPECIAL proof (semi-smokeless) had been conducted (Crown over NP & Cordite 70, 480 MAX). Since all proof was for black only (unless "NP" was also present), there was no need for a specific "Black Powder" proof mark. It was not until the 1954, and subsequent, Rules of Proof that proof for semi-smokeless was assumed and specific marks for black powder proof were finally introduced ("Not Nitro" for London, "Black Powder" for Brum).

British proof rules and marks are extremely simple. All the books make them extremely complex, largely because they don't explain them as a system. I've never found one that didn't contain errors. The Standard Directory of Proof Marks by Wirnsberger is fairly decent.

Note that "semi-smokeless", "smokeless", and "nitro" mean the same thing.
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"Serious rifles have two barrels, everything else just burns gunpowder."

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