500mm lens for RZ67 fires when DoF lever pushed

Hello,

I have the Mamiya Sekor 500mm f8 lens for the RZ67 system. When I use the DoF preview lever the shutter fires. The only way for me to open the shutter on the lens is to move the two prongs on the back of the lens.

Ive never seen then on any of my lenses for my camera before. Any help would be appreciated.