Didn't get a chance to pattern the shotgun to estimate what the fixed chokes might be, but I did test fire it and shot a round of trap.

First shots (safety first): https://youtu.be/CD-p9GfVK-g

Seems to shoot really well.

Also got a response from Rivolier:
"Dear Sir,
This shotgun was sold by our firm in the 50.s .
The gun is a French design, made in France, prooftested in the city of Saint-Etienne.
I do not believe we actually made it. Most probably only the barrels and the finishing.
Will try to see if I can found some more info but we have very little of archives from this period."

Not sure what to think of that response, seems a little confusing.

The hunt for more information continues.