trying to figure out what this is

[The Long Ranger]

I found it cleaning out my Grandparents attic. Grandpa says he found it out hiking back in the day. That makes it at least 40 years old but I think it really is much older. On the base of the blade is engraved "U.S.
1863
F.S.S"
the other side has something like ...Helmford... in a circlualar pattern, the rest has worn off. F.S.S is on the guard as well.
What really is getting me is that the blade is around 18 and a half inches long. Any clue as to what this is or what is worth?

 

[Zeke66]

FSS is the Federal Supply Service, gear for federal agencies