Maybe it's an original plug. We call hydrants "plugs" sometimes. First arriving Officer will have an engine "find a plug" or tell them where it is. The term "fire plug" dates from the early 1800s, when water mains were made from wood. The fire department would head out to the fire, dig up the cobbles down to the main, then chop into the main so that they could place the hoses from their pumpers. When finished fighting the fire, they'd seal the main with -- you guessed it -- a "fire plug." Then mark its location. The next time there was a fire in the neighborhood, they'd dig up the plug and not have to cut into the main.