The tool pictured here is a Millers Falls compound miter vise. Apparently it was another of the models patented by Langdon that Millers Falls manufactured. While the tool lettering calls it a miter box, it is actually a rather small tool that would apparently be held onto the board to be cut to guide the saw at the correct angle, unlike most miter boxes which themselves hold the wood to be cut. The angle of cut is adjusted in one dimention by pulling on a spring loaded stop pin and pivoting the sawguide assembly until the pin engages the appropriate stop. The angle from vertical is controled by loosening a wingnut and adjusting to the desired angle and then retightening the wingnut assembly onto the small metal bar that holds this part of the fence at the right angle.
I first saw this vise at a flea market but did not buy it. I then kicked myself for four years until I ran into the very same vise at a tool show and purchased it. This is the only one I have ever seen and I have not seen a reference to it anywhere though I imagine some Millers Falls catalogue must list it. According to the embossed lettering it is a Millers Falls Number 41 Langdon mitre box, as you can see in the photos below: