These heavy 4 inch markers have a 2 inch long hook sprag on the back to
set it into wet concrete when the sidewalk is poured. Many soda companies
supplied them to areas near schools and high pedestrian traffic for sidewalk
safety markers in the 1930's. It would sit flush with the top of the
sidewalk, and provide a permanent marker for separating pedestrians from
motorized traffic, and at bus and streetcar stops. Schools would use them as
cue up points to form a line for the bus.
Most of the surviving markers like these were removed during WWII for the metal. Any that made it into collections are usually substantially worn on the top. The foundry that produced them ( National Safety Marker Co, in Pontiac, MI ) was demolished just after WW11. The back (underside) says, Patented 4-11-33 National Safety Marker Co. Pontiac Mich.