I have an ICS manual on tramming that describes how the shoes and wedges are set up for final machining on a planer. The book instructs to lay out "proof marks" 1" away from where the final face of the shoe or wedge will be. Then the shoe/wedge is held in a "chuck" that has 4 centers that suspend the wedge by its proof marks, and machined on a planer.

I'm curious what other methods there are to hold shoes and wedges, square up the proof marks, indicate the 1" away from the proof marks, and then machine the face of the wedge? Never done this job before, and am wondering about modern techniques, especially those that can be done on a mill, for shops that might not have a shaper.

On a mill, anything that holds the work in place and at the correct angle relative to the table will be sufficient. Proof marks, a scribed line, or just indicated in, held in a fixture, there are many ways to get the job done within steam engine tolerances. I think the old school reciprocating rather than rotary tools needed a firmer grip - you could vary the depth of cut but not the linear VS tool feed rate.