GOOD DAY ALL!!

FOUND IT!! Or I am fairly confident I found it... You were lucky to have all the parts... I only had the spring, and that was probably because it got trapped on the other side of the opening... It looks to be a flow valve for the two halves of the pump... It looks like a restriction for the larger pump gears... Yep, I pulled it apart again to confirm...

This is where my spring ended up... There is no machine work done to this side of the port... Flow goes thru this port to the filter, but the spring just couldn't get turned the correct way to "go with the flow"... So, it stayed right here...



This is the other side... See the spring, barely?







Okay, I tried multiple times to get a closer picture... Wouldn't work for me, stupid phone!! But the guesstimated measurements are about 23.45mm for the largest diameter, 21.8mm for the next one, and I didn't measure the pass thru hole... I'm going to guess it fits that three legged plug you have... Cup, spring, plug (legs up) is the order... Like you have it in pic three... Now, about that washer... There is no oil residue on it, so I am going to say it was just in the wrong place at the wrong time to be in this line-up... With the ring at the base of that cup, I'll bet the outer diameter of it is about 21.XX mm... Or something like that... And that line is about 14.8-15.2mm from the bottom of the cup... There is also a beveled edge on the inner hole... Probably mates up to the angle on the plug...



Now, is it needed?! I doubt it... Can you have too much flow from the pump?! Is it used to divert pressure to another item?! Possibly... That huge valve is right there... I can't get the cap off to pull the valve out... It's a 16.5mm allen or something like that...

I hope this helps!!

Cheers!!