I grew up from 5 - 23 years old in Manchester.
I have no idea what this chick movie is about.
All I know is that we never called it "Manchester by The Sea", that seems a bit too pretentious.
I'm not sure if that was its official name the entire time, it might have been changed in the 80's, not sure.
The 1% in Manchester were 50%.
Edit
I just looked on wikipedia
Name
To prevent confusion with the nearby and much larger city of Manchester, New Hampshire, the name of the town was officially changed in 1989 following a close town meeting vote that year and an act of the state legislature passed on September 25, 1989