VW water pump. The Volkswagen water pump lasts a long time. Don't over tighten the alternator belt as the added tension creates a burden on the water pump's bearings. If water is dripping out the hole in the bottom of the pump, it is bad and you need to replace it. This part uses the 30mm hub. You must go and measure if you are unsure. 1981 was a split year between this pump and SN1322.
Bolts tend to snap off in the VW water pump housing when you take them apart. Tap them repeatedly before attempting to loosen them. Consider ordering a water pump housing one when you replace the pump to be safe! Also, don't forget your water pump gasket, two thermostat housing O-rings (part #1333), and a set of bolts (part #16079).