If we look at the assembled parts outside of pump to gauge overall size, the copper washer sits a few millimeters below top of pump when housing is removed.
The copper seal seats internally around where the rubber O ring is outside.
The next piece shown below is fixed being anchored by a slot. Calculating distance from here until the copper seal face indicates they are held in situ, this part i believe is a piston sleeve.
Looking from below this is a very smooth bore like surface for piston travel, the exit hole sits beneath the tail of X piece.
