This is an outstanding key-hole cover from the Eastlake period (1870-1890). It appears to be cast bronze, although it may be cast brass. Both mediums were used by pre-1900 hardware manufactures.
The escutcheon measures 2 1/4 inches vertically and 1 1/4 inches horizontally. The key-hole opening is 5/8 inch long. Screws for mounting will be included.