So what your saying is earlier cars came with teak trim and this one had the trim variation to the walnut?
Not sure if I would call it a variation since we don't normally refer to changes the providers or Ford makes when they change a style or design of a part on the assembly line with that term. The earlier color/finish was not longer available or installed.
On buildsheets they use a code DEV which I think might be decoded as "deviation" located after the interior code boxes (2 of them)>
Kevin Marti's The Mustang & Cougar Tagbook shows a list of interior codes and the deviation versions also According to that reference the extra "A" on the interior code of Mach I with black interiors is sort of related to the dash insert finish as is indicates that the darker door panel insert was to be installed. He chooses to use the term "variation"