part # 26 in the attached link:
Oil Pump
note that it is a 997 part number. it is beveled where the original was square. my understanding (worth what you pay for it on the internet) is that the original was binding, and the role of the piston is to increase oil pressure at start-up. a 5-minute job at your next oil change.
it's an interesting exercise to go through the parts catalog and look for superseded part numbers and 997 part numbers - aos for example ...