The blade is triangular and really isn't rusty. The hand gaurd is studded and has some surface rust. The sheath or whatever is in really fantastic shape too.
What did I find?
US Model 1917 “Knuckleduster Trench Knife. Originals would have been made by Landers, Frary & Clark of New Britain Connecticut.
Can’t tell if it is original. There are a lot of replicas.