Sorry I can't supply the list with an explanation (at least we now know there's at least two of us wondering...).
Yet I can contribute the following bit, found in the the N9Txx Engine Workshop Manual where, for the '86 model vehicles (all with N9TE engine),
"5" appears to stand for "505 Turbo GL Edition" with Manual 5 speed
"6" appears to stand for "505 Turbo Sedan" with Manual 5 speed
"Z" appears to stand for both of the above + "505 Turbo SW" when with ZF's 4HP22 auto trans
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