ANSWER:

In the late 1800s, both state and federal laws emerged to protect animals in the United States. One such law passed in the 1890s required that horse drawn delivery carriages carry a spare horseshoe. In the case of a lost shoe, the driver — hopefully also competent in driving nails, too — could apply the shoe without shaping it. This “spare tire” could get the horse through the working day until a blacksmith could nail on a replacement shoe. This is a bar shoe version of a “coachman shoe.” 

SOURCE:

This shoe and description are courtesy of Pat Reilly (Grad Dip ELR), chief of farrier services at the University of Pennsylvania’s New Bolton Center as part of its Podological Collection.