Check out the photo of the components in Aaron Lau's "Simplicity" pen kit.
The transmission has external screw threads like the one you're looking to replace -- whether the threads match (or any other critical dimensions) is a different question, but the shape is approximately correct.
Look at woodnwimsies.com there replacement trans for Monet/Gatsby looks close but same as above the size mite be off....but it does have outside treads....
I picked up a few of those trannies from Woodenwhimsies and the outside diameter of the thread is 6.8mm with a .5mm pitch. I would bet a 7 x .5 tap would work for these trannies for custom work just fine