Dutreuil was a dealer in the Champagne area in France (Epernay & Chalon s/ Marne). Thad Scott had a H&H Royal copy with that name on it a couple of years ago.
As often the case, the guy most likely just sold the gun and did not build it. It would be have been built for him in Liege or St Etienne. The proof marks will say which. Anything is possible about the quality but the brand value is basically worthless.
Best regards,
WC