Ricoh XR-2 lens release button.

I picked up a brace of old Ricoh cameras on the wekend.Overall nice condition.The XR-2 has a broken lens release button that I'd like to replace with one from a Pentax K-1000.
For the life of me I can't figure out how to remove the old piece.Took off the lens mount ring and now I'm stuck.Anyone have a diagram of the disassembly or a web site I can check?

Thank You