I presume you mean this one:http://www.gunsinternational.com/guns-fo...un_id=100650513
This one might not be as simple as popping the barrels off, and looking at the grade stamps.
To begin with, notice the name "Darne" on the opening lever. It is done in plain, block letters, not the more stylized name that came around 1910 or so, and that is a big hint we are looking at a really old gun. As are the remaining bone pack case colors on the sliding breech. Around 1946, cyanide case colors became typical, but only on the R10, until the R10 itself was discontinued, not long after some changes to French proof laws, around 1964. The breech itself has nuances of styling that are more in line with an 1894 patent Darne R, rather than the 1909 model that moved the safety lever to the breech block itself, and off the water table. As noted above, it is an R, the later patent gun, still produced today.
The monobloc barrels are another sign of a graded gun. People could and did spec monobloc barrels on lower grade guns, like a Halifax, but, it did not happen often.
Before Darne started putting round grade stamps in the flats of the barrels with a tiny "Darne" name in them to identify grade of gun, they used little stars. The quality of a gun that had one or two stars, denoting R11 or R12 grade, respectively, would not necessarily match the quality of a later R11 or R12 gun with the usual Darne in circle marks.
They were usually much, much, better. An R11 Darne was a high grade gun, circa 1908 or so, and the model A or model C guns were the lower grade guns in the lineup. Neither the model A nor the model C were sliding breech guns. Notice the screws for the ejectors are timed perfectly, on both sides? This went away in later years, as the factory tried to ramp up and cut production costs, and then came back in the Bruchet era.
I'll confess that I haven't seen the barrel flats, but, if it has the little stars instead of typical Darne quality stamps, I'd speculate an early model R12 produced in the first decade of the last century.
The dimensions and barrel length show that the company didn't build them all with short and lots of drop stocks. This looks to be a decent, usable gun, even if it is quite old. If the scale at G & H is accurate (makes me wonder when they advertise a Darne model R sliding breech as a "boxlock") that practically screams heavy weight barrels, a desirable option from that era.
I'd be very curious as to condition of the bores in a gun this old. Many are heavily pitted. It could also have been proofed with powder S instead of powder T, as both powders were offered for proof after powder T was introduced in 1900. If the gun is still in proof, loading it is a non issue, for the most part.
I would love to see this one up close, and take a few pictures and notes.