I found out the answer when I worked at Greenfield Village in Michigan. A Railroad buff might know.
Elijah McCoy was the man behind that phrase. He settled in Ypsilanti, MI and worked as an oiler for the Central Railroad. He invented an automatic lubricator for trains. As years passed other lubricators were invented but, railroad people would ask for Eli McCoy’s lubricators by saying; “Give me the real McCoy.”
Elijah was also the inventor of the folding ironing board and the lawn sprinkler. Now you know what we mean when we say; “The Real McCoy.” :)