TH 350 ID Codes:
69-72 Buick = Model & Calibration stamped on 1-2 Accumlator cover.
73-81 Buick / Chevy = Model stamped on Governor cover and Calibration stamped on right side verticle surface of Pan. Chevy Pan code only indicates Plant, Year, Month, Day of Month & Shift.
About 80-81 the TH350C got the Calibration Ink stamped or Stickered on the Bellhousing. Easliy removed when cleaning.
Vin # = Chevy, stamped on right side of Case just above Pan rail towards front.
Vin # Buick, stamped on left side of Case near Manual shaft.
If you get a Trans that's been rebuilt, the Gov, Acc. & Pan cover could have easily been changed.