Song-plugger
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A Song-plugger is a piano player employed by music stores in the early 20th century to promote new music. Typically, the pianist sat on the mezzanine level of a store and played whatever music was sent up to him by the clerk of the store selling the sheet music. Patrons could select any title, have it delivered to the song plugger, and get a feel for the music before buying.
[edit] Source
"Autobiography of a Father," Romney Lyle Pearce
George Gershwin was a song plugger in his early years, as was Ron Roker.