Focus tension on old Zeiss microscope.

Hello,

The focus adjustment knob from a Zeiss surgical microscope is in the photos here. I'd guess the scope was built in the 1950s or '60s.

The shorter setscrew here was in the knob when I began work on it. The longer screw is one I made. The knob threads onto the focus adjustment spindle. The female thread in the knob is visible in the photos.

Tightening the knob on the spindle increases tension on a clutch to prevent the head of the scope from sliding out of focus.

Tightening the setscrew in the knob should lock it on the spindle to hold the tension setting but the setscrew has no effect.

Obviously the setscrew would spoil the spindle thread by landing directly on it. The setscrew must push on an intermediate part with a thread matching the spindle. I don't understand why it doesn't work. The longer setscrew is still too short?

Not an electronic problem but similar knobs were used on some vintage high end electronic equipment.

Ideas?

Thanks, ... Peter E.