The part number for the water pump gasket (goes between pump and engine) is 21114AA051. This supercedes the older 21114AA05
0.
On svxcess's "Tune-Up parts" list (MS word doc, link in post #9 above), he also lists P/N 21116AA020 "Water Pump Seal." This is a smaller part that appears to go on the side of the pump closer to the car's centerline.
In the drawing (from SubaruParts.com) it looks more like a bracket — reference number 21116.
(click thumbnail for larger image)
What is it, and does it need to be replaced?