Trying to figure out why the N designation would be unique to fastbacks or convertibles?
Likely laden weight since it was greater for both those cars when compared to a coupe with the same suspension packages and what the engineers penciled out
Body style is just one of many things that can affect originally assigned front or rear springs. For the fronts even the option of PS can make a difference. Again something that would add some additional weight to the front end of the car
The last printed version of the MPC lumps all the following together for a non- K code application for replacement spring purposes . No mention of handling package cars or any other original variations. IE the possible reasons for all the different code variations
C4ZA-C, E, M, T
And would the 6E denote a June 66 assembly?
No as the article in the Library reminds us - they were using the Julian calendar pattern for the dating of these
So in your example the 45th and the 65th days of the year.