i am about 90% sure this is a piston out of a brake master cylinder. most likely early 1940s. it is the right size and all the old master cylinder pistons have the small holes on one end and the groove on the other end would have held a rubber washer that created the seal. the hole in the end is where the rod from the brake pedal goes to push the piston. i am still trying find a picture to confirm it but i am pretty sure that is what you have