There is a small brass washer in package with the float needle valve a drawing I found shows it inside the seat is this right as it don,t fit or does it go under the seat
Is this link showing the washer you are talking about;
If so I don't recall seeing that brass washer in my L4 valve but then again I reused my old valve.
Side note; make sure the old gasket is cleaned out before installing the new valve.
Looking at Langs illustration, could be a soft copper gasket that goes in the hole first then the valve body and upper gasket. Could be that the illustration in the discussion was just shown in the wrong place.
Yup that's it
Without it in hand I would say under the seat. Give it a try and see how it fits, if the upper gasket does not want to seal I would just leave it out.