This small back plate has a locking apparatus that when moved into place will prevent the spindle of the doorknob from turning.—or lock the doorknob. Since it is in the flashed copper finish, this piece dates from 1900 to the 1920s.
The beveled plate stands out from the surface of the door about 1/4 inch. The plate measures 4 3/4 inches long and 1 3/8 inches wide. The hole centers are 3 7/8 inches.
This photo shows the plate with the lock over the doorknob hole. The plate could also be mounted the other way, with the lock under the doorknob.
This is a very uncommon piece of flashed copper hardware. We have it priced to sell. A rosette could be used on the other side of the door. Screws will be included.