From Rennlist, nice write up by Erkka Vilhuer. Differential adjust Vilhuer.pdf
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This is late style numbering system taken into use in '88 MY.
Code:
123456789ABCDE
G2813 1J 00501
123 = Manual gearbox for 928
45 = '87-88 US model gearbox
7 = Open diff, LSD would be 2
9 = Model year, '88
ABCDE = Sequence number within gearbox type and MY,
501 is first normal gearbox number
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Differential adjust calculation, donated by Barry Orlando.