This small full mortise lock dates to the Eastlake era (1870-1890). The box is heavy cast iron, while the face plate is cast brass or cast bronze. The lock maker is Branford.
The box measures 3 inches long (or deep) and 1 3/8 inches wide. It is 1/2 inch thick. The opening for the door knob spindle is 3/4 inch from all three sides of the box. The thick face plate measures 3 inches long and 7/8 inch wide.
The lock is in good working order. This one is a rare find.