Depending on how you do it, you don't even need a left handed screw. In the image above, the whole screw moves forward through both nuts. It's only the slight difference in pitch that causes the movable nut to move. The more compact way to do it is with concentric parts, and you turn the central "nut" that's both internal and external threaded, to move one of the screws. I think Lansing sold a diff mic that was actually made for them by Starrett. Burleigh sold one for a while that was made by Slocomb. Newport sells one, and Thor Labs too! None are cheap.