http://www.network54.com/Forum/509674/thread/1151937032/Excellent+article+on+rebuilding+the+Diana+Giss-action+pistols.....
See link above. Seems to me there was also a photo essay of rebuilding a model 6 posted here before but I can't find it at the moment.
The washer functions as a shim and spacer, it is critical to getting the action to work correctly. Its thickness may need to be adjusted when you replace the bumper. The chunky bit is the rear piston bumper and you most definitely cannot shoot the gun without it.