Need a ruler laid next to it. Could be anything from a small safety pin to a diaper pin to a mail bag pin. Need measurement.
Also, electrolysis would help get the rust off, but since it's such a small item, watch it carefully while in electrolysis.
It looks like a safety pin. As we use different types of stylish pins on our hats,blezers,sarees and many other things, so might be it's a conventional kind of safety pin used in old times.