Your gun is an 1894 patent Darne model R action, built after 1900, by someone other than the Darne company. Regis Darne changed his actions in the early 1900s, selling the older patent alongside the new ones for a time, before he discontinued his use of the older guns. These same early actions were then sold to other makers in the white, barreled, and proofed, but never marked with the Darne name.
Your gun looks to be a nicely finished 12 gauge, with 65mm chambers, and single proof out of the St. Etienne proofhouse. French single proof is nothing to sneer at, plenty of margin for safety. If the RST 2 3/4" ammunition is fairly low pressure, I wouldn't worry about using it in your Darne to at least try the gun a bit. If the ends become ripped off the fired rounds, save them for a different gun, and use a 2 1/2" load from RST, next time. Generally speaking, a 2 3/4" load in a 2 1/2" chamber won't cause any trouble if it is within about the same pressure as the 2 1/2" load the gun was designed for.
If you just cleaned the gun up recently, your Dad did a great job of storing it all those years. It looks to be in good shape.
I confess to having never seen the M*M stamp before, and do not know what it represents. The good news is any clay pigeons or birds you use the gun to pursue won't know, either, and it won't make a difference in shooting.
Any further questions you might have, post them here and we will do our best to answer them.
It looks to be a fairly nice shooter. Do enjoy.
Best,
Ted
