It's a main spring for a sidelock.
I use them all the time in my work.

What lock exactly I can't say. The hook on the end means it goes to a tumbler with a stirrup in it.

That kind of set-up came out in the late flint period, but was common all through the cap-lock period and into the cartridge age. Most good double and single side lock rifles and shotguns have this type of spring even today.