Mystery Message

[rogerkirk]

This morning whilst commuting from Fremont to Billerica, I noticed an interesting (and cryptic) message scrawled in the dirt adorning the back roll-up door of a white trailer (NH plates) headed south on Route 495.  It said:

Free 107.3
N/P

Any guesses as to the significance?  I'm drawing a blank.  The tractor and trailer were virtually devoid of any significant markings to identify the city of origin.