Perplexity goes along with Grok:
What you likely have is a Gustavus Masu London-made 12-bore hammer gun, and the rib address “10 W Wigmore Street London” matches period listings for Gustavus Masu / Masu Brothers at Wigmore Street in the late 1860s–1870s. Contemporary references also show that a Masu gun with that address has been recognized by collectors, including a 12-bore example described as circa 1870.
Maker and dating
The “Mash” inscription is probably a misread or worn version of Masu; the surname is documented in London gun trade references at Wigmore Street, including “10 Wigmore Street, W.” and earlier “3a Wigmore Street.”
That points to a gun from roughly the late 1860s to early 1870s, before the move to later addresses.
What the gun style suggests
A London 12-bore hammer gun with 29-inch Damascus barrels fits the style of a high-quality black-powder-era sporting gun. Damascus barrels were common on finer British guns of that period, and surviving examples are often valued for their pattern-welded appearance and craftsmanship.
The “great dimensions” you mention matter a lot: if the gun still handles well, that can make it very desirable to a collector or vintage-gun shooter.
Stock and furniture
A beautifully figured stock with a professionally done extension and an inlaid steel buttplate suggests careful restoration rather than rough alteration. On an old London gun, a good extension is not automatically a flaw if it’s well executed and the rest of the gun remains original and tight.
Those details can preserve usability while still keeping much of the gun’s visual appeal.
Shooting value and caution
Damascus-barreled shotguns can be safe to shoot if they are sound and properly inspected, but chamber length, barrel wall condition, proof status, and load pressure matter a lot. British Damascus guns are often associated with black powder or low-pressure ammunition, and a vintage gun should not be assumed safe with modern high-pressure loads.
A competent gunsmith should check the bores, proof marks, chamber length, and action tightness before any shooting.
What I’d do next
The most useful next step is to look for:
maker’s name on the locks or under the rib.
serial number on barrels, action, fore-end iron, and trigger guard.
proof marks on the barrel flats.
any patent or address wording beyond the top rib.
If you share photos of the rib, lock plates, barrel flats, and proof marks, I can usually narrow the maker and date range much more precisely.
