Mount adapter for 20" cinema hd display

I bought a mounting arm for my desk, one for iMac and one for second display which is a 20" cinema hd aluminum display. I bought the VESA mount adapter but it does not seem to fit this display. Once I take the leg off with the two screws on leg, the mount does not match up. It is to wide for the screws to tighten or match the holes. Is there a different mount adapter I need? I assumed one adapter worked for all displays. Any help or suggestions would be appreciated.