That's the part, but I assure you, it's not diesel only. The car in question is a late 1979 production Trans-Am ordered with Bright Red paint, WS6, rear window defroster, radio delete and grey decal package. At the time of ordering, the original purchaser also specified grey wheels. The engine cooling radiator has the regular transmission oil cooler with 1/2 -20 fittings in one tank, and 5/8-18 fittings in the other tank to accept the engine oil cooler lines.
My thinking is the non WS6 cars with the 2.41 rear axle did not receive the engine oil cooler, only the WS6 cars with 3.23 axle ratio.