CptCurl:

Your post is certainly well received. Yes, I had the same thought as to how off-topic this has gotten.

As for a comprehensive reference work on British proof marks, I'm afraid I'm at a bit of a loss. The problem with most of them is that they leave you dealing with the "pieces" (which one of your later questions highlights) rather than illustrating the use of the marks as a system. "The Standard Directory of Proof Marks" by G. Wirnsberger (translated by R. A. Steindler), is pretty good, but has a few errors. The Proof Marks appendix in Nigel Brown's "London Gunmakers" is very good, but this book is expensive. Practice helps a lot. Check the reference books against guns of known provenence and you get the hang of it pretty quick. Pay particular attention the changes to the marks in 1904, 1925 and 1954.

No, I don't think these barrels were simply reproven for Nitro. As stated in my original post, were that the case there would be two full sets of marks, the original BPEs and the new Nitros. I've handled many re-proven rifles over the years and this is the way I've always seen it done. There is only the one set present.

I understand what you mean when you say "They have black powder proof". However, this is not really correct. The interlaced GP surmounted by a crown is simply London's definitive proof mark, and is not specific to black or semi-smokeless. (Yes, I know that many reference books erroneously describe this mark as London's definitive black powder proof mark.) It does not mean that barrels so marked were originally BP proof. Until 1955, there was no such thing as a "Black Powder Proof Mark". As hard as it may be to believe, until the 1954 rules, ALL British proof was black powder proof ONLY - UNLESS the definitive proof mark was accompanied by a mark indicating that "special" (semi-smokeless) proof was conducted ("NP", "NITRO PROOF" or "NITRO PROVED"). The 1954 rules reversed this and proof is now assumed to be semi-smokeless. The same definitive proof mark is still used in exactly the same way. For BP only guns, London now marks "Not Nitro". The marks on your flats indicate full, original nitro proof for a .450/.400 3.25" NE, 1925-1955. The marks of any brand new rifle in that caliber proved in London during that period would look exactly the same.

One other piece of information might be enlightening. So it originally weighed 8 lbs, 7 oz as a .400 BPE. Just right for a BPE, 1 to 1.5 lbs light for a .450/.400 NE. I wonder what it weighs now?

A very Merry Christmas to you and yours, Roscoe.
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