I've looked for period instructions on how to sharpen these and came
up empty in the past. I'd be curious to know if anyone can dig
anything up.

Lacking any concrete evidence, I looked at the NOS bits I have and
found that they appear to be simply ground across the flute at an
obtuse angle, maybe 135 degrees. I've done the same with my bits and
it works well. They are simple tools and pretty hard to screw up.

Josh