Yes these are used to machine the jaws prior to machining
With the hydraulic chuck it has a tendency to splay due to workholding pressure.
I have found that by turning a bung to the middle diameter of the jaw stroke and placing it at back of chuck then turning a counter bore in the front face of the jaws then removing
the first bung and placing the larger disc with a hole for clearance of the bored jaws diameter will
counteract the splayed jaws .This also loses some of the clamping area of the jaw to the workpiece.
Very labour intensive requiring some large rings for large diameters you can see why these tools would be a labour saving device.

Regards Phill