Couple of things.
1894 is the correct patent date for the sliding breech R model double. To the very best of my knowledgs, the R model Darne was never produced with damascus barrels. 1897 is very, very, late in the ball game for damascus barrels, hammers, pinfire, and single shots in the French gun industry. The French, unlike the English, took to steel barrels and hammerless designs, like a baby duck takes to water. I've seen a few model C lifting breech guns with damascus tubes, but, most are steel.
I didn't see the word "Darne" anywhere on the gun. That design appears in none of the Darne catalogs I own, which, go back to 1900.
I think whoever wrote the blurb for the description has a few facts out of order, or, is outright incorrect. It may be a fine gun, may be a Darne, but, from what I see, there is simply no evidence of that.
It pays to research a bit, before you buy.
Best,
Ted