Cast iron rosettes were often used behind the black porcelain knobs during the Victorian period. A black patterned back plate like this added class to the porcelain knob. The key hole was then covered with a patterned escutcheon.
This rosette measures 2 inches round. The opening for the neck of the door knob is 1/2 inch. There are grippers or teeth on the backside to dig into the wood. Screws for mounting will be included.