Holger
The nylon is a spacer/thrust washer. It lubricates the meeting surfaces of the rudder and hull when there’s upward thrust (the rudder being buoyant if it’s not waterlogged (at least on the C30 it’s buoyant.)
I don’t know what the top of the rudder post is like on the mk-1.5 but on my 30 with the exposed emergency tiller cap next to the lazerette there’s also a thrust washer under the cap to take downward thrust.