This is a favorite area of mine so I spent a little time digging out some history. It's complicated, and I'm not sure I have all the details sorted out correctly yet but here's the gist of it. This part of the Benton Range was a logging area with a number of camps and forest roads, including the North-South Road, which the CCC completely rebuilt in the early 1930s along with the dam on Long Pond.
The USFS Long Pond Tr per se also dates from the early 1930s. It initially ran W from Long Pond (near the current dam) past a ruined logging camp at 1 m. and ended at the Black Mountain Tr at 2.1 m. There it joined DOC's trail to Black Mountain, which then joined what was called the Southwest Tr at the logging camps. DOC's Black Mountain Cabin was near the former junction on what was then its trail. DOC abandoned its trail and, presumably, cabin by 1940. The Long Pond Tr was formally abandoned sometime around 1955.
(P.S. Not a Dartmouth alum, just a local yokel who loves the Benton Range.)