1959 383 for an American Chrysler would MR383, if memory serves. (RB block)
When looking at old cars, especially well worn ones, never assume that what you find on that car was on the car when it rolled off assembly line. One never account for the tastes of car owners as various interior and exterior bits and pieces are added or removed.
A 1962 Chrysler 300 letter car was 300-H and 1963 was 300-J.
The 1962 Chrysler would look like a 1961 model ,minus the fins. The 1963 had completely new styling, based on Exner's proposal for the S seres 1962 Imperial. The 1963 looked like a 1964 model, only (again) minus the fins.