IIRC
The top stud is a metric thread and I believe it a a 4 or 6
Get thee to a hardware sore and purchase a small bolt that will fit the threads.You may as well pick up a tap too.Get that bolt about an 1&1/2 inches long and a washer and nut to fit it.
drill a hole in a block-of wood the size of for the tube but don't glue the tube in it.
Then as driften said use it like you would a gear puller.
Put the washer and nut on the bolt, slide them trough the bock of wood and then screw that into the stud.Here's where the tap may come in.
The threads in Te AC pen from Woodcraft would not allow the bolt to go in afar enough to get a bite.
If this is the case, retap Te hole in the stud.
Put the assembly back in again and snug the nut up to the washer.Make sure the hole in the block is centered over the stud.Now as you continue to tighten up the nut you will pull the whole assembly out of the pen.
Here, you have a choice.
I have not had much luck reusing the transmission and have purchased replacements but I hated this kit so much because of this problem I stopped making them.(I think I may still have a few hanging around.)
It's too bad because it is a traditional kit and good looking for those who like a Parker Style pen.
It is for this reason I switched to the perfect fit and never looked back.