Stephen, The gun was proofed in St Etienne. The barrels are stamped “double epreuve//“ (double proofed) and the proof mark verifies this.
The Marking “M Serre PT” is not clear (possibly meaning “tested or hardened for PT” - “a St Etienne” - “in St Etienne.”
There is a mark on the barrel lock…”AUHOLEREGI” or variant… and “CURTIEN” or variant. If you can provide a readout on these words, I’ll research them some more. This might be the barrel maker.

Re the marking “Roturier, Ste. Foy la Grande.” - this looks like this is the seller or perhaps the firm which assembled the gun. This is normal in French guns. Little hardware stores all over France marketed guns assembled from imported parts...sometimes not even putting a name on them...or if they got enough aristocratic patronage, they'd add a name....

Sainte-Foy-la-Grande is a commune in the Gironde department in Aquitaine in southwestern France on the Dordogne River 30 miles east of Bordeaux. (Aquitaine - think Gascony and D'Artagnan from 3 Musketeers). I believe it was once two towns…the Port Sainte Foy de Pinuilh and Sainte Foy…they apparently merged in the 19th century into Sainte Foy la Grande. (believe Porte Sainte Foy was the head of navigation on the Dore River and was quite prosperous for a time). The Dordognee River runs due east from Bordeaux then up into the Midi and forms the border between Geronde Department and Dordogne Department. There is a canal up the river starting at Ste Foy.
There apparently was a very well known company there the Etablissmente Roturier in the 1920’s-50’s which manufactured tools, assembled bicycles, repaired motorcycles, tractors and sold hunting and fishing equipment, etc., typical French hardware store serving a region. Monsieur Jean Edmond Roturier….may have been the founder or a descendent. I can’t find much about Messieur Roturier or his company. However, in 1976 the “Maison Roturier,” listed as an “Armurerie” (a gun store i.e armory), was established by Jean Marie Edmond Vircoulon, identified as an former employee and associate of Roturier… Another post indicated M. Vircoulon took this firm or at least the name over in 1955 and ran it for 54 years.
Vircoulon retired in 2009 and his shop at 41 Rue Victor Hugo was closed. The name, however, ultimately was taken over by a young 26 year old protege Bastien Pierrot. His shop is at 19 bis de l'avenue Leclerc à Pineuilh (part of Ste..Foy la Grande). Here is an article about him.
http://www.sudouest.fr/2010/09/17/l-armurier-renoue-avec-la-tradition-187242-3038.php
Pierrot studied at the armorer’s school in Saint-Etienne, spent 5 years in Italy working for famous makers of shotguns, spent another year in London in sales and repair of shotguns. His store “Maison Roturier” sells rifles, carbines, shotguns and shells. He also repairs guns. Since he was in London, he probably speaks English (and by the way that whole area was owned by the British crown up to about 1420 (Elenore of Aquitaine married (2nd marriage) Herni II, Duke of Normandy and King of England = the 100 Years War, i.e. battles of Crecy, Poitiers and Agincourt, Ste Jean d'Arc, etc) and is very pro-anglo/American).
Vircoulon is now the unofficial historian of the area. I think he is on Facebook. I’m sure if you wrote to him and sent some pictures, he could tell you more about the gun and the history of the Ets. Roturier…. My father was killed in Normandy; I was tracking down the locations of all the photos found in his camera and my experience is the French are very helpful in such matters.
By the way “roturier” in French means “a person not of noble birth” or a “commoner.” As an aside, in 1983 upon returning from Africa my wife and I drove up that road into the Midi going from Bordeaux to St. Chamond and St Etienne. The Dore is beautiful...
Final note: Maps.google have this shop still at 41 Rue Victor Hugo (see sign saying "peche"(fishing) "chasse"(hunting) in the window (MBK is a brand of moto cycle). (I believe also by that time Vircoulon was involved in real estate which may explain the photos by the door); Possibly Pierrot moved back to the original premises? But on the other hand, google doesn't update its french photos that often and this photo might still be from the mid 2000's before Vircoulon retired. From the cars seen in the maps.google street pans, the latter is probably true.

We need to see photos of the break key and lock. and other parts of the gun. Thanks.
Gene Williams