The AD is very likely an inspector's mark rather than a maker's mark.
Yes, Whitworth did number their tubes.
Littlegun does not list a N. Jacquet gun maker so he was likely the lock maker.

Odd for a gun of this quality to have no clear indication of the maker.
Littlegun states that Charles did not take over Masquelier Freres until 1925.
It is possible the gun was made for a dealer by Charles, but for some reason never marked by the dealer, which was usually on the rib.

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