Steve:

Yes, the .450/.400 3 1/4" was a popular black powder express round long before the Nitro Express version appeared. The .450/.400 3" Nitro Express was designed expressly for Cordite and was never loaded with black.

Also, "nitro proof" would not automatically mean that it is a Nitro Express. Some rifles were specifically proved for the "Nitro for Black" load, which was loaded with semi-smokeless, so they may bear a nitro proof mark but are, technically, black powder proof.

I keep up with the values of British DRs fairly well, but don't pay as much attention to the Belgian guns as I should. The name on the gun doesn't impart a price premium. Jansen is rumored to have been primarily a retailer. Assuming full nitro proof, reasonable weight, excellent bores, and no problems in evidence, I imagine it might be a good value. It certainly isn't a bad looking gun. The best guy to ask would be J. J. Perodeau. He trained and worked as a gunmaker in Liege and is likely to be more familiar with DRs of this make than anyone else in the US that I can think of.
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