Just to make sure we're talking about the same thing. I'm referring to the left side, outer most valve spool seal bushing. Not the plastic piece. Not the centering spring, plastic bushing.
Currently I have the slightly fatter washer that came with the ram kit test fitted. It fits well and is not galled like the make-shift one I made.
My reference is the diagram you posted below.
Mine is just plain missing. Valve appears to be newer. Maybe it was never installed during the rebuild.