The hinge pin on a Charlin is located at the end of the opening lever, and rotates in the receiver as the lever is cycled open and closed. You have to take the stock off to see it, and you have to drift a pin to remove it.

Stan,
I'm not positive what you mean, exactly, with the "build a better mousetrap" bit. The above advertisement is from the French magazine "L'illustration" in 1932 and makes mention of the fact that more than 120,000 Darne guns are in service in France and abroad at that time. Are you saying they were not successful with the design? That Purdey or Boss or Fox, or LC Smith or whoever, had higher production, and were therefore more successful? Or are you perhaps unaware of just how many Darne guns are out there, and, that they are still actually being produced?
Do tell. Can you think of a company that produced double guns in those quantities by the early 1930s, and then kept on doing it?
I like this little note, too:

Note they make mention of how many dealers they have- 500 worldwide. And how they were pretty much dominating pigeon shoots in places that mattered in 1952.
Better mousetrap, eh? Every gun has it's strong points, and weak points. None of them are the end all that will simply work best for every human being. But, eliminating the hinge pin was a significant achievement. It is an uncomfortable fact that every hinge pin will wear, and will need to be serviced at some point.
But, not on a Darne.
For the record, I like a Charlin, I just like a Darne R model better. I'd take a Charlin that fit me before I'd take any US built double, for example. A Charlin doesn't have a through bolt in the wrist like a Darne, and should be easier to have fitted for cast and drop then a Darne, but, everyone has their preferences. I had my 12 Darne stocked to fit, so, that was that.
Have a great day, Stan.
Best,
Ted