A picture of a sliding breech gun with the extra two shot magazine beneath was on the cover of American Rifleman many years ago. I don't recall the maker's name but he was Belgian.
Levi was the Belgian builder. The pictured gun gun is/was in a museum in Belgium, that Roger Barlow had toured, and I highly doubt there are more than a few in existence.
I'm not sure what kind of information the OP is looking for. Watch for short chambers or pitted bores. If a Darne doesn't fit, don't buy it, bending adjustments are difficult. The safety lever on an actual Darne R model can be flipped around to the other side. Most of the clones have a rocker style safety that the earlier Darne patent guns used, it can't be relocated, and if you can't get used to a Greener left safety, you won't like a Darne, either.
20 gauge guns are few and far between, typically. I have used both the semi pistol grip and the straight grip, and find that I much prefer the straight grip.
If you need repairs, Kirk Merrington is your guy.
Most of the complaints on Darne guns can be traced to the gun simply not fitting. "It kicks" is often poor fit, or heavy loads in a light gun, or, both. They can have heavy trigger pulls, but, Kirk has worked a few down in the past. I am not enamoured of sub 4 lb triggers on field guns anyway.Trigger correction is more likely to be successful on a V or P grade gun. They aren't especially fast to reload, but, most Frenchman (and, me) have never been on a driven shoot anyway. Larry goes on those junkets, maybe when I'm as old as he is, I'll think about it. Or, not. I can reload my R about as fast as a guy can reload a conventional ejector gun, but, I'm not going to do it all day, either.
I wish you the best of luck in your quest. If you need instructions on how to take one down (hint: no tools are used) you can PM me, or post here and someone will help you out. In the right hands (not Larry's) a 16 or light 12 gauge Darne can be a quite efficient upland gun for birds.
Best,
Ted
