That doesn't seem right, but I don't have a better reference to offer. All 928's are 8 cylinder and the automatic vs 5 spd is in the first letter G or A.
G28/XX YY Zz NNNN
G28, A22, or A28, with the split between A22/A28 I think in the middle of 83.
/ is just a spacer
XX is tranny type and doesn't match to year model changes.
YY can be missing or show differential type.
Zz first digit seems always 1, second digit I think is standard vs optional gear ratio
NNNN serial number.
This applies to the VIN and
AFAIK not to the tranny.
digit 3 - 0 means 1980. 8-1978, 9-1979, A-1981 etc. Letters I/O/Q are not used. S-1995.
On the motor id, M28/XX, the XX is a sequenced model number, but the second digit shows 5 spd or auto, and is often 1 = 5 spd, and 2 = auto.