Chapuis makes good-quality shotguns and double rifles and its catalogues can be found at www.chapuis-armes.com
It makes several guns with the addition of sideplates to provide room for decoration and to give the impression of a sidelock.
The website has an email contact section, so you could ask about the model of your gun and the year of its manufacture.
English is not routinely used in France, but I have found that an enquiry in French, however bad, will often produce a response, unlike one in English.
Compose your message and translate it using Google Translate — it is not perfect but the recipient will at least understand it.
It is not possible to value any shotgun without seeing it.
Valuation is a complicated equation whose main factors are quality, condition, rarity and mechanical soundness.
I can’t find any information about Chapuis shotguns