If your car is a real 1975 360 Dart Sport, the serial number would start with LM29L5.......
According to what i found, there was 1,043 built total.
Sunroofs and rear fold down seats are rare on all of the Dart Sports, let alone a 360 car which is pretty rare by itself.
I do have a '75 Dart Sport 360 fairly plain jane car that needs restoration myself, BTW.
I used to have a '74 360 sunroof rear fold down seat car which i know is really rare too..........
