I don't remember this gap being this big when it was new. And I saw other post mentioned about it as well.
This is the old teflon washer Po.32 . you can see it has been squeezed pretty badly after all these years.
I wonder why it's not included in the kit.
I've found a few OEM online, found out the thickness of 0.5mm. Then I measured the washer, with ID. 13mm OD.16mm CS.1.5mm
Instead getting the original 0.5mm thickness, I bought 1mm. and it turned out fine. No more play and gap. maybe just 0.1mm left.
I think a 1.5mm thickness should completely remove the gap after tighten the screw. but that won't be good for rinsing.