My WAG:
It could be as simple as: Kevin is including "Trim Variation" nomenclature, to more recently generated Marti Reports that show 3 digit interior trim codes. In the Mustang and Cougar tag book, there are footnotes next to most every 3 digit trim code. 1967-73, there are instances of suffix code 1, A, B, C, & E, all with footnotes. The footnotes have notes and the word "variation". Some variations are explained.
Only my guess, please don't shoot the messenger.