There are clones, and then there are clones. That is pretty clearly an 1894 patent Darne R action, likely sold out the back door of the Darne works after they were selling the 1909 patent Darne R model. But, the engraving, wood, checkering (notice the "seashell" treatment on the ball of the semi-pistol grip) and the step up to mono-bloc barrels all speak to a maker who gave a damn about the guns that had his name on them. I'm pretty sure that is the best example, quality wise, I have ever seen in a Darne clone, period.
I have handled three or four examples of guns by this maker, and every one of them was a cut above the competition, in execution and quality of materials. I wish I knew more about them, but, when I was in St. Etienne and asked the people at the factory about A. Bayle, they hadn't heard of them, so it dead ended for me, right there.
I would bet that upon inspection, you will find it has heavy barrels, and a lot of material in the mono-bloc, and that the chamber lengthening, if done by a professional, is not an issue.
Best,
Ted
