Mortimer Site Strider CPM Dumper Pivot Information required

[DADumper2]

My son in law has a dumper which according to decals on skips is a Mortimer Site Strider CPM. The pictures look remarkably like those posted by Richard666 in 2010.
The machine has Hamworthy axles with inboard brakes and a Lister ST3 diesel engine which I have recently finished rebuilding.
I have been using Winget manuals as a general source of info for repairs to the dumper but they do not seem to apply to most recent problem which is the pivot arrangement between front and rear frames.
In the attached pictures 1020531 and 533 there is a bright yellow ring like a large washer 140mm OD 6mm thick which fit behind the end cap on the horizontal pivot pin beneath the hydraulic hoses. At the weekend I noticed that this "washer" has slipped down by almost 25mm and was projecting out of the joint out on the lower side.
Before I start to strip down the swivel I would like to understand what I should find and was hoping that someone may have information or a manual for this part of the machine.