Concur on the early interwar date. I've seen a couple that were alleged to be pre-WWI, but no explanation for that dating was provided. The design is overall more of that seen on interwar doubles. The problem with dating French doubles is, unless it ran through a proof house that stamps dates (e.g., in Belgium, German or Austria) there's no definitive way of knowing. You might be able to suss out a date from the combination of makers' and mechanics' stamps, but I'm not aware of any compendium of such that would make it possible to do so (unlike over in the German-Austrian room, where a couple guys know not only who the mechs were but also whose in-laws they were).
I have a similar gun, sans sideclips, that I and everyone else characterize as a guild gun. I think a guild-er is what you have here. Chances are, if it were a Manufrance it would say so on the gun, and the same for the Robust.
All in all, it's a nice hunting gun and one you can carry all day. $400 seems a fair value, depending on the bore condition.
