Again, if one looks at the manifests for the harvests brought into St. Louis during what we normally consider the fur trade era buffalo hides, or any other fur dressed or otherwise, make up a very small percentage of the hunt. Beaver was king. What beaver that were along the Platte, Niobrara, Grand, Kaw and other tribs of the Missouri were gone quickly....probably before 1820. Thus the Ashley-Henry hunt of 1822 to "penetrate the Rocky Mountains". One must also remember that it was not only waters that drained into the Missouri. The Rocky Mountains feed the Missouri, the Columbia, the Colorado and if one goes on into Canada, the big river that heads NW....doggoned if I can remember the name of it. Thus the area ultimately drains into the Gulf of Mexico, the Gulf of California, the Pacific Ocean and the Arctic Ocean.
As to 50-100 true Mtn. Men I believe that is a very low number. There was more than that killed in the generally accepted 1825-1840 period. Jed Smith lost almost half the superior number on one expedition. After the fall and early winter hunt the hunters/trappers wintered in secluded or not so secluded valleys in the mountains. Either with friendly tribes or in "brigades" of their own. Friendly tribes for the Americans mostly means the Crow and Shoshone, mountain Indians at that time. They stayed in those valleys, or holes as the Mtn. Men called them, so they would be close to the hunting grounds for the spring hunt. I cannot recall any account of them wintering out of the mountains and on the plains...neither am I infallible. Also, there was more than just one company competing for the fur trade. There was almost always at least two American companies, Hudsons Bay and before their merger with HBC, the Northwest Company. That doesn't include the free trappers who traded where they wished. If one says there was probably no more than 50-100 free trappers...I could maybe be convinced of that. Most hunters/trappers/Mtn. Men were employees of one company or another and as such absolutely were not issued a Hawken rifle and it is doubtful they could afford one. The American fur trade companies were the early version of the company store and by their own folly most employees were kept indebted to the company. I do not know if the British companies were set up in a similar manner. I am inclined to think not.
Another point to be considered is that the heyday of the Hawken rifle was not during the peak of the fur trade. It came shortly afterward. Nor was 50 cal. normally chosen or mentioned.