Some pictures of the fine tuning mech from the
hp 3310A function generator, also used on 3310B, the shaft has the plastic disc moulded on to it, the same as the control knobs and was probably made the same way.
The top surface is angled, the one spec pictured has worn down quite a bit to have a flat on the edge.
For comparison, the shaft with moulded disc from 3310A on the left and the larger one from the 200CD tube oscillator on the right.
A failure mode of these (200CD), it has cracked, will probably fall apart same as the controls do.
And finally, the complete fine tuning spring mech from the 207A sweep oscillator.
David