Can someone tell me what this is?

If you take the gun down, there will be proof marks and other insignia on the underside of the barrels and on the water table - CLEAR pics will help us help you

From WIKI:

Manufacture d'armes de Bayonne (sometimes also called Manufacture d’Armes Automatiques de Bayonne) was a French pistol manufacturer. The company was founded by Léon Barthe in 1920. It survived the German occupation during World War II, and after several changes in ownership wound up being a subsidiary of Fabrique Nationale of Herstal sometime in the 1970s.

MAB made parts for FN under their ownership, but eventually was forced to close in September 1982.

Click to expand...

"BARTHE ARMLES" Then under that it looks to say Bavonne?

Click to expand...

It actually says, "Barthe Armes Bayonne"

Other than that, I do not know the history of the company.

French company that survived the German occupation, and then pretty sure it became a subsidiary of FN...

http://unblinkingeye.com/Guns/MABHist/mabhist.html

BTW

The place to ask is here:

http://www.shotgunworld.com/bbs/index.php

While there are lots of very knowledgeable people here, salmoneye is correct about the place to ask a very specialized question like this. Post closeup photos of the watertable proofs and we may be able to tell you more... the photos you posted just show a generic SxS.

generic, but it is a boxlock, has side clips

underneath the engraving, it appears to say, "Importe De Belgique" - which should translate (roughly) to Belgian import

pics of the proof marks will bear out whether my translation about it being Belgian are correct - they will also tell us the gauge, bore size, choke and chamber dimensions

Euro shotguns of pre WWII manufacture are a real can of worms and often a wonderful opportunity.

Almost certainly this came here in a dufflebag, ca 1945-46. The splinter forend and straight grip indicate this was meant as a "Game Gun", a light,agile bird shooter. It's a boxlock, not the more prestigious sidelock. However, side clips ( the little projections on the receiver "Fences" that bracket the barrels and reduce play) were hardly ever fitted on junk guns. This looks like a decent grade of shotgun.

The bad news. Almost all of these are chambered for ammo SHORTER than we use now, and using that modern ammo runs pressures well up into red line territory. Short ammo is available but not in wally world bulkpacks.

Some of these old doubles have enough metal to tolerate rechambering to 2 3/4" laods like we use and some do not. And some few have 2 3/4" chambers already. And some have engraving that says "Non Pour Le Balle", which means NO SLUGS.

A good going over by a smith is called for here. Once chamber length, safety and headspace is established, get some appropriate ammo and have fun, in your ancestor's memory.....

TTT

My previous response may have been in error, so pull the forearm and see what proof marks are there , that will answer a lot of questions.

Manufacture D'Armes {MAB} was founded by Léon Barthe...

The gun in question is marked 'BaRthe', not BaLthe...

JABC

It is best to seek out the shorter shells for these older guns.
Rechambering them is an option but the higher pressure & longer shells will quickly wear the actions to the point of uselessness.
These guns were certified proofed for a reason in merry old Europe.