This is an antique nickel plated mortise or barrel bolt with a thumbturn, for use on one side of a door to lock it. These are traditionally mounted above a doorknob set on a bathroom door or as additional security on a storm door. These are not accessible from the other side of the door.
The barrel bolt is 2-1/2" in length overall with a 1-7/8" backset. Backset is the measurement from the edge of the bolt to the center of the thumbturn. The thumbturn is 1" diameter and the backplate is 1-1/4" diameter.
This item is in good condition overall with some light wear to the nickel plating on the thumbturn. Backplate is in better condition with very little wear.