From Grok:
This is almost certainly a gun by Gustavus Masu (also known as Gustave Masu, Masu Frères, or Masu Bros), a London gunmaker with Belgian roots operating at 10 Wigmore Street, W., in the late 19th century.
The inscription "Mash .10 W Wigmore Street London" is a very common way these appear when the engraving is worn, stylized, or partially obscured—the "u" in Masu often looks like an "h" in old script or photos, and the address matches exactly ("10 W" for 10 Wigmore Street, West). Masu guns frequently have ribs engraved along these lines (e.g., “G. Masu, 10 Wigmore Street London,” “Masu . 10 Wigmore Street London,” or similar variations with the brothers/frères name).
Background on the Maker
Gustavus Masu (born ~1824 in Belgium, died after 1876) was active in London from the mid-1860s. He had ties to Liège (a major gunmaking center) and London. His firm operated under names like Gustave Masu (1864 at 3a Wigmore St.), Masu Bros (mid-1860s), and then Gustavus Masu at 10 Wigmore Street from around 1869–1880s/early 1890s.
Masu was more of a retailer/finisher who imported or bought actions/barrels (often from Belgium) and finished/sold them under his name. His guns were generally good-quality, mid-to-upper trade pieces—not quite "best gun" level like Purdey/Boss/Holland, but well-regarded for shooting and often nicely finished.
They won recognition (e.g., honourable mention at the 1862 Great Exhibition) for quality.
Typical Features Matching Your Description
12-bore hammer guns (often sidelock, non-rebounding or rebounding hammers) were common from this maker.
29" Damascus barrels — Very typical period length for English game guns. Beautiful Damascus (twist or laminated steel) patterns were a hallmark; these were nitro-proofed later on many surviving examples for smokeless powder use (though always check proof marks and have a gunsmith inspect before shooting modern ammo).
Great dimensions and gorgeous stock — Masu guns often had nice walnut with good figure. Many had well-done stock extensions (common repairs on older guns to adjust length of pull) and steel buttplates (sometimes inlaid or engraved).
Actions were typically straightforward English-style hammer guns, sometimes with features like rotary underlevers on earlier pinfire-converted examples.
Value and Collectibility
These are desirable collector/user guns today, especially with nice Damascus barrels and original condition. Value depends heavily on:
Overall condition (barrel wall thickness, bore, action tightness, stock cracks).
Engraving quality and original finish.
Proof marks (London or Birmingham, nitro reproof if present).
Serial number (helps date it more precisely).
Comparable Masu hammer guns in good shape often sell in the low-to-mid thousands USD range at auction or through specialists, with premium for exceptional wood/Damascus. They shoot well for those who enjoy classic English guns.
Recommendations
Look for the serial number (often on the action, barrels, fore-end iron, etc.) for more precise dating.
Have a qualified gunsmith (ideally one familiar with English antiques) check it thoroughly—Damascus barrels need careful assessment for pitting, dents, and suitability for modern loads.
Resources: Books like London Gunmakers by Brown, or forums like the Internet Gun Club (IGC) and DoubleGun BBS have threads on Masu guns with photos.
If you can share photos of the full rib inscription, locks, action, proofs, or serial number, I could help narrow it down further. Nice find—these have a lot of character!
