Modern Vespa

Answered my own question.
Oil pump gasket ripped on me so gotta order one of those may as well through the bolt and flappy washer thingy in as well.

Yeah might as well replace it. That screw clocks the oil pump sleeve to the input and output holes in the sleeve line up with the ones in the pump body. It can be easy to bugger it even if you think you have it lined up. I think it would leak even if the o-rings were fine if it wasn't a good fit. Might add a drop of sealant to the threads when putting in the new one.

I seized an engine because I clocked the sleeve wrong. When you push the sleeve into the bore, there wil be resistance from pressure. Keep pushing until the pressure bleeds off. Then the sleeve will stay where you put it.