This particular insulator is called a battery rest, designed to help insulate banks of batteries from the ground. Hemingray produced these for both the National Battery Co. and the U.S. Light & Heating Co. These particular pieces have a distinct waffle-pattern base, presumably to help keep them from sliding around.
1 13/16" H x 2 1/2" W
• NATIONAL BATTERY CO. • U.S. LIGHT & HEATING CO. • U.S.L.