Closer inspection of the rear of the lens mount revealed a ball-bearing mechanism for the aperture actuation lever. There was a locking ring over the ball bearings and the ring was too tight, causing the ball-bearings to bind.
Even closer inspection showed that there had originally been a drop of glue or paint to keep the ring from tightening.

I loosened the ring to the original position and applied a new drop of paint to hold it in place. The aperture then worked perfectly.