It's a 2 piece snap ring. The upper portion comes off by popping the "ears" towards the thread on the upper pivot bolt, then you can pull the lower portion out.
Is the upper pivot sticking? Some Campag 11 spd RD had issues with the upper pivot sticking if the fixing bolt was over-tightened. Loosen the pivot bolt slightly and it should pivot. I have a Chorus RD on my Ritchey that will stick if the fixing bolt is torqued to the recommended 10 Nm. If it is torqued to 8.5 Nm it works fine, but anything over that and it starts sticking.