Thanks for those last tips. I was hoping that in this case a normal tap would do the trick because the hole for the depth adjustments is open at the bottom (it ends in the space for the screw that holds the blade to the side (and I imagined a bottoming tap would be harder to find).

I very much like the trick to use a hardened bolt and dremel tool. From what you said, I imagine that a 10-24 bolt might work, even if it is a 3/16 – 24 hole. Might be better even, I guess, if all I can buy to replace the orignal screws are 10-24 threads anyway.

I remember seeing an episode of the woodwright shop (on the PBS website) in which Roy’s blacksmith friend made threads by hand. I can’t remember in detail but I think it was similar to what you describe in using an adjustable die. Come to think of it I think I’ve seen some more modern adjustable dies on the local ‘ebay’ (one life one: https://www.marktplaats.nl/a/antiek-en-kunst/antiek-gereedschap-en-instrumenten/m1737807843-compleet-antieke-draadsnijmachine-oster-mfg-co-cleveland-in.html?previousPage=lr) and wondered about those. They seemed like intriguing tools (if these are what you mean), but not really what I would need or use.

Whenever I need a strange bolt or screw I dive into all the threading info, but always end up a bit dazzled.

I’ll let you know how get on with my plane (but could be a while – these projects usually progress in cycles of activity and inactivity…)

Cheers!

Sebastiaan