Hmmmm, felt like I kind of stalled the discussion. Too bad and not my meaning. To try to illustrate what I was after, I quickly made a 2D sketch.
It is based not on a ruler but on two bars, say about 6 mm dia and some various sized brass blocks about 25 mm high and 10 mm thick. Should be sturdy enough. Thickness probably OK if 8 mm.
The two end pieces are pinned to limit the risk for twisting. At the right end the end block is wider and carries one of the depthing pegs. The other larger block slides sweetly on the bars without any slop at all. This block carries the other depthing peg.
To achieve a micrometer adjustment, the smaller moving block to the left of the left peg block, has a locked in thumb screw working into a thread in the adjacent peg block. With a spring between them it will be easy to adjust minutely the gear centres.
Locking the depthing pegs is done like a height gauge and locking the peg block and adjustment block is done using a cross "bar" nipping the slide rods like the arm locks on a GHT pillar tool.
This would be easy to make and if the rod holes are drilled and reamed before parting the brass bit, the distances will be exact. Probably simple to make as well.