3.55 Safe-T-Track in 1970 GTO could be STT or STT HD , up to the buyers (or dealers) knowledge.
Have 1970 Invoices reflecting both STT and STT HD in Manual Trans D-Port cars without any axle ratio option (3.55) .
Also have a 1968 3.55 Posi from a 4spd GTO that is not an "N" housing.
Don't know how-why on that one - its just that way
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