I'll throw in a WAG. Setting device for adjustable parallels.
One fixed stop on the side of it and one adjustable stop that was adjusted to make it square the moving rack.
End/side of the parallel set up against the stair step of the width you choose.
Move the rack back and forth to bring the measurement you wish to set to under the adjustable side stop while accounting for the parallel end position differences you would encounter upon different adjustments..
A quick tool for bench setting them without using/dedicating a Vernier or a micrometer, not needing that much precision perhaps. Would explain the no need for calibration as well, used as reference only.
Seems very excessive for that purpose, but what the heck, its just a WAG!