Where does this go?

[ScumRat]

OK did some googling
got the part No there
https://www.xs650shop.de/en/electric/1079/altenator-rotor-key

And with that the dimensions there ( its in the list below )
https://hvccycle.net/yamaha-generator-rotor-woodruff-key-90280-04005-00/

Fits with your measurements 4 x 13

View attachment 232034

Thanks so much, I’ll look into it when I get back to the house

 

[joebgd]

I believe it's more likely the rotor side than pump.

 

[Wirenutt]

Some PMA installs don't use that key any more. They rely on the taper fit of the shaft to hold the new rotor where it should be (fat chance, lol). Not a very good set-up at all, if the rotor moves, the timing changes. Yet another strike against these PMAs.

Just goes to show some engineers don't have much experience in the real world. Also, the engineers who graduated with a 1.9 GPA have to work somewhere!