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Afaik the markings for Nitro proof were introduced by the British proofhouses in 1904, several years after NE cartridges came into use. So any gun using Nitro Express cartridges made up to that year will show only the old black powder marks, though it is intended for cordite.

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That's not correct. The British rules of proof of 1887 and the later rules of 1896 both provided for methods of proof and markings for nitro proof.

As an example, here's a photo of my 1901 Westley Richards .303 clearly showing nitro proof:








For a discussion of this, please refer to Gerhard Wirnsberger, The Standard Directory of Proof Marks at page 103, and also to Nigel Brown, British Gunmakers, Vol. 1, at pages 265-266.

Curl