For those of you who wonder what an M800 is like inside, here is an exploded view.
Top row - Inner cap, cap, clip, top retaining ring
Middle row - barrel, section, nib collar with the metal ring, feed and nib
Bottom row - grip knob, drive spindle, connector bushing. anti-rotation bushing, piston shaft, seal
I did not try to remove the trim rings. They're too firmly attached.
Clicking on the picture opens a larger image.