In the image above you can see the crush washer (part 7f). This needs to be removed. Insert the end of a flat screwdriver under the lip of the washer, and gently slide it out.
Now the crush washer rests on a metal collar, the collar holds a very small cylinder of metal. This cylinder, from what we could gather, when the engine is running pushes against the spring and allows a small amount of fuel up the lines, through the injectors, and into the engine. I have been repeatedly warned not to disturb this but to my surprise the collar and little cylinder were quite happy to slide around inside the pump - they can't really not move, because the pump isn't horizontal anyway, its tilted to one side.
I think what may have happened is that someone has torqued the delivery valve back down, not taking care to ensure the collar is correctly centred in that little well, and the little cylinder hasn't been able to fully move - which would probably lead to damaged seals inside the pump, and a damaged delivery valve. With respect, its easily done as 7g moves around a fair bit, and the spring is carefully balanced - so take great care here.
Anyway, we removed it just to have a look:
There appear to be no keyways or anything, the pdf file (I will link it at the end) just says "observe the installed position". There is a groove along one edge, we just compared it to the collar on valve 5 and aligned it in the same direction. The little cylinder is flat to the top edge (with a small serial number stamped in), the bottom edge looks like a phillips screwdriver blade - probably so the diesel can counteract the rotational forces applied on the top edge when it pushes against the spring.
Obviously you don't have to remove this collar or the little cylinder, so don't unless you're curious. If you do remove it, make sure it goes back in the correct orientation, and clean.
Put the new crush washer in place, and slide your new o-ring over the thread of the delivery valve. Your fingers will be covered in diesel and oil anyway, but if they're not just make sure the o-ring is wet with oil/diesel, to ensure a better seal while torquing down.
Carefully put the spring back in the pump, and ensure the collar (7g) is centered in the chamber.
Screw the delivery valve back down
with your fingers, waggle it back and forth as you do so the collar, crush washer, and spring are all forced to align themselves correctly inside the delivery valve (the inside of the valve is slightly fluted to aid this).