On my Vickers pump, there were two internal o-rings on the backing plate area. I used a big ol' C-clamp to compress it, then pop that retaining ring out. Believe me, this tested my wits BIG time.
I had no reason to remove the shaft, my front seal wasn't leaking (I planned to replace front seal) and after my PS pump ordeal (don't ask)... I will cross that bridge when I come to it. Better to leave it alone.
My SDL pump needs a rebuild but I haven't figured out how to remove it yet let alone tear one apart...