Chat GPT is also on the Masu bus:

What you likely have is a late-19th century London-retail Belgian-built sporting hammer gun sold by Gustave Masu / “Masu Bros.” of Wigmore Street. The rib inscription you’re reading as:

“Mash .10 W Wigmore Street London”

is almost certainly:

“Masu, 10 W. Wigmore Street, London”

The firm operated from 10 Wigmore Street from about 1869/1870 through the 1890s.

A few important things about these guns:

Masu was associated with Liège, Belgium gunmaking, while retailing through a fashionable London address.
Many were very well-made guns, often with excellent Damascus barrels and attractive stocking.
They are not generally considered “best London guns” in the Purdey/Boss/Holland sense, but good examples can be superb shooters and very elegant sporting guns.
The Belgian connection is not a negative in itself. Liège produced huge numbers of fine sporting arms in that era.

Your description actually sounds unusually appealing:

29" Damascus barrels
“great dimensions”
gorgeous stock
steel buttplate
well-done extension

Those are all signs someone cared about the gun over its life.

The key to understanding value and quality is in the details you haven’t mentioned yet:

1. Proof marks

Remove the barrels and photograph:

barrel flats
water table/action flats
underside of barrels

These marks will tell whether:

it has Belgian proofs,
London/Birmingham reproofs,
chamber length,
black powder vs nitro proof,
approximate era.

That’s the single most important clue.

2. Action type

Is it:

Jones underlever?
top lever?
rebounding locks?
back-action or bar-action locks?

That helps narrow date and quality level.

3. Damascus pattern

Fine “stub,” “crolle,” or “Bernard” Damascus can indicate a higher-grade gun. The fact that you called the barrels “beautiful” is promising.

4. Bore and wall condition

Condition matters enormously:

dents?
pits?
wall thickness?
on face?
looseness?
original barrel length?

A gorgeous old hammer gun can go from a wall-hanger to a highly desirable field gun depending on those points.

Dating estimate

Based on the address “10 W. Wigmore Street,” I’d estimate roughly:

circa 1870–1890

If it has exposed hammers and Damascus barrels, that fits perfectly with the period.

Collectability/value

Masu guns are somewhat under-the-radar today, which means:

they don’t bring Purdey money,
but knowledgeable hammer-gun enthusiasts often appreciate them.

A clean, mechanically sound, attractive Masu hammer gun with good dimensions and strong Damascus can absolutely have meaningful collector and shooter interest.

If you want, upload photos of:

the rib inscription,
full gun both sides,
action,
proof marks,
barrel flats,
Damascus closeups,

and I can probably tell you:

approximate date,
Belgian vs English proof history,
likely original quality level,
whether it was a higher-grade gun,
and a realistic current-market value range.