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That's a tough one. So, there is no aperture ring at all, due to it being controlled by the camera. The last photo with EF back, does it give infinity? If there was some leeway and infinity adjustment screws, a spacer say 2mm thick shaped like a "C" could be sandwiched in between the lens body and EF back. Then, an extension of the aperture fin out thru the opening could control the aperture in a non-detent manner. Make sense?
What is the orange ring made out of?