Detached knob on Ica Ideal iii

I have an Ica Ideal iii, but unfortunately, the right "squeeze-pull" knob that allows the bellows to be pulled out has been screwed out.

I can't see any hole under the cover plate for the knob to screw into, presumably because it has moved out of place. I do see a spring (coil-type) with a loop at its end, which is anchored to something in the middle of the slider.

I don't think I'm missing any parts.

Any suggestions on how to fix this would be appreciated.