You will need a couple of rods
1 that will go thru the transmission holdler to knock out the nib
from the nib end another rod preferably larger diameter to knock out the transmission holder.
Unscrew the cap that holds the clip.
Drill a hole in a block of wood that is larger than the neck of the clip holder but smaller than the body of the barrel.
Press this piece out on a drill press or lathe or if you can rig your pen press ,that will work also.Getting the black ring out is tricky,
Angle one of the rods to catch the edge of the black sleeve inside the tube.
Holding the barrel tightly( remove rings and use a jar "grabber" to get a grip on the barrel) hit thte rod "smartly"
This will take a few attempts but you will see it working its' way out.
You are not the first this has happened to.
Again remove jewlery from the hand you are using to hold the barrel.
(I learned from experience(read mistake and dented pen))
Any set of long punches, a long bolt or the like will work.
No need to buy expensive things to take a pen apart.