A few quick comments. More details will have to await pictures; FAB500 will likely comment too.
Manufrance early records were lost in a flood in the 1930's? However, there has been a lot of research on early models:
Jean-Claude Mournetas wrote very well known books on Manufrance including the "Ideal" and the "Robust." These volumes included charts to date the guns by model number:https://www.leslibraires.fr/livre/1...l-de-la--jean-claude-mournetas-pecari-edhttps://www.amazon.fr/Fusil-Robust-J-C-Mournetas/dp/291284861X
The charts are posted on this line p.10 & p.11:https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=488406
He also wrote on "The Simplex" here which I'll try to take a look at: (Canvasback on this site might have the book; he had both the Ideal and Robust books; Gil - GLS - certainly had both) - Note "roi des fusils à un coup" - King of the single shot:https://www.eyrolles.com/Accueil/Livre/le-fusil-simplex-9782912848048/
This advice from another site: "If you remove the butt stock recoil pad or plate the date of manufacture will be stamped on the end of the wood stock..."
I would be willing to translate selected articles from the catalogs if you can identify them. On the other hand once photos are posted, FAB500 might point us more to the correct year.
Incidentally, this also from another site - cannot vouch for it but it sounds right:"Simplex shotguns are made by MANUFRANCE since 1908. They are still made. These guns were widely used when using conditions were bad or very bad:sea side,equatorial Africa,south america(Guyana...) because they were sturdy and inexpensive.
Let me tell you one use :the gun was loaded with a cartridge cut just over the wad ,then a wood arrow of the bore diameter with iron point was introduced by the barrel muzzle and the gun was fired in order to kill ....elephants!! Of course the shooter had to fire point-blank and run or climb a tree very quickly!! Just to say the guns were sturdy ones. Another type of users were people without a lot of money and youngs.
First this shotgun was only produced in 12 and 16 gauges but a few years later in 20 and 24 gauges for youngs. In the other side,10 gauge (for waterfowl),and rifled barrels in 375HH ,305 WCF...,the action was strong enough to accept these cartridges. After 1920,the barrel had a tight full choke,before the barrel was less choked. In 12 and 16 gauges the chamber length was 65mm during the 50 first years of production,then it was 70 or 76mm.
The early models had either an outside hammer or were hammerless.The first type was stopped in 1930.The safety appeared only during this year.
The butt pad was iron made during the first years,then horn . Bakelite appeared around 1950. The gun is still made:
12 gauge, 3" chamber, 80cm barrel length
20 gauge, 3" chamber, 76 cm barrels. Both have a beech stock and forearm."
French gun term added to the French-English dictionary:
Monocoup - single shot