The number is stamped on the bottom of the differential housing. Should start with 915/63 then the serial #. From my info 1980-84 have 915/63 on them. If it's a 1976 and up it has a 8:31 r&p (72-74 have 7:31)
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1970 T1 W/MassIVe 2913cc RAT/?EFI? w/direct fire (very soon) and 915 trans
1962 SC 1776cc SP 944NA brakes, 993 wheels
VKG