it goes at the bottom of a well to keep the water in the pipe , a check valve . if not for the foot valve every time the pump cut off the water would fall back down the pipe and you'd loose prime.
all pumps have to have a foot valve or you will have to prime with each use.
I'm not even saying it is a foot valve , I'm just saying it looks like one. could also be an air release for a steam boiler or a anti siphon device or any number of things.
one clue would be the threads , are they machine threads or tapered threads. tapered threads get wider as you go to create the seal.
This appears to be a brass air release valve, now I need to determine what it could of been on at this location of my dig from late 1800 to early 1900’s, there has been nothing current on the property since owning land from 1952 that I can associate this valve coming off of. Unless it fail out of a truck or tractor. Any guesses would be appreciated. And to the point of threads , they appear machine threads not tapered. Thank for everyone input. Any clues to machinery used with valve?