3978,
Sens is a small town between the Burgundy and Champagne regions.
Mr Desmaret was likely to be a dealer, not a maker. It was customary for the dealers to have their names engraved on the guns they were selling. These guns were usually made by the St Etienne or Liege trade. The proof marks will show which one. I bet the engraving is actually something like "Armr." for Armurier (gun dealer) instead of "Arms" which is not French.
Regards,
WC