There are three 'feedscrews' on a standard ML7. The long one under the edge of the apron, running the length of the machine, doesn't have a scale as standard, but there is an extra calibrated round handle at the tailstock end. The other two are on the cross slide for the toolpost, and on the toolpost slide itself. Both have a standard bevel scale, but fixed. So, it may be that your ML7 has been modified over the years, and it is certain that an adjustable scale is a useful mod to have. So, can you tell us a bit more about the position etc of your problem scale, please?
The normal scheme for an adjustable scale is to have a spring holding the disc with the calibrations against a disc fixed to the spindle. Friction moves them together, but allows the scale to be zeroed so you can return to a given setting easily. If you take one of them apart, do it inside a polythene bag, and then you won't spend three days looking for the spring…
Regards
Tim