[T77911S]

had to look it up.
its for "bucking" trees once they are down.
watched a video of a guy using one. pretty cool.

stand the saw its end and place the hook at the bottom in the top pic on the tree, then with the saw straight up, the weight (25lbs) pushes the saw down as it cuts the tree.
basically you are using the end of the saw to cut which is normally a no no with a regular saw.

saw a few warnings about kickback with this thing. don't think I would regularly use it although I may restore it.

in the bottom pic the 2 round parts that have no paint are the clutch (the bigger one) and a smaller gear. it looks like the chain speed is reduced, not sure why. perhaps as a safety thing.
that part also has gear oil in it. not bar oil, that is upper left