This is an antique Corbin interior mortise lock. An interior mortise lock is a lock which is used on an interior door that has been cut with a pocket or hole into which the lock is to be fitted. Interior mortise locks commonly include a doorknob hub, through which a doorknob spindle is installed and doorknobs are mounted on each side and secured into place with pins or set screws; and a keyhole which accommodates an old-fashioned bit or skeleton key which in turn throws the deadbolt to lock and unlock the door. mortise locks have specific dimensions including backset, the measurement from the edge of the faceplate (the strip that shows when the mortise lock is installed in the door) to the center of the doorknob hub; and spacing, the measurement from the center of the doorknob hub to the center of the round part of the keyhole. The dimensions were not standardized in the 19th century so it is important to note the dimensions of your door prior to purchasing a mortise lock. Please find the item size in the Dimensions tab to the right.