This back plate probably came off of a front door. It is larger than the normal size. The treatment is flashed copper which was very popular from 1900 through the 1920s. The plate is stamped steel, then coated with copper, and finally flashed or sprayed with a black lacquer. Through the years, this treatment has not stood up very well. Therefore, it is difficult to find flashed copper pieces in good condition.
This 10-inch by 2 3/4-inch back plate is in good condition. It has some surface scratches, but the flashing is not disturbed. The beveled plate projects from the surface by 3/8 inch. There are four holes (in the corners) for mounting. The distance between the door knob hole and the key hole is 3 1/4 inches.
This is a handsome back plate.