Ok I think I an deduce what might be happening. You did have a one key system, but when you removed the bolt, the flanges that push against the ring, sheared off (probably because they were rusted through).

At this point the best approach is to jack the arm off from the back. My tool of preference for this job is a set of Jacob's Chuck #6 removal wedges, (cost under $10.00 in the USA) but You can sometimes simply pry it off with a crowbar. If using a crowbar, you have to get the point all the way to the spindle, then rock back against the face of the BB cup.