The seal is definitely riveted on; I had a 622 with the identical seal. What I did was to cut away the old seal (no going back); this allowed me to push the washer toward the piston head. I then used a punch and hammer to roll the expanded end of the rivet back down and remove the washer. I supported the free end of the rivet at the same time. Kind of a pain, but that was all I could think of. Then replaced the parts and re-expanded the end of the rivet. Good luck!
Paul in Liberty County