As I tend to like roughish guns and guns that may have been messed with a long time ago, if I had that I'd clean it up (like you have) and put those sights back on it. I am wondering if someone fitted those sights for use by cadets or scouts? With the rear being none adjustable for elevation there is no blade to break. That might tie in with the hole in the cocking lever, and like you i am wondering if someone fitted a sling?
The top of the cylinder looks like it had a scope rail tack welded onto it at some point later on.
It is never going to be worth much but the sort of gun you wont mind using at all.