Michael, then usual disassemble sequence is to knock the tip end out with a punch inserted through through the tranny. However, that looks problematic with yours.
If it were mine I would probably try to drill a clear access hole through the old tranny to clear that ragged scrap out of the way.
You can make a nice holding clamp out of a 10-12" piece of scrap pine 2x4. Drill a hole through the center of the 2x4(through the 2" dimension) about 3-4" from one end that JUST allows the pen barrel to pass through it. Starting at the end the hole is closest to, rip cut the 2x4 through the center of the drilled hole but not all the way through the length of the 2x4. Stop the rib about 3-4" from the other end. Now you can place your pen barrel in the hole and use a c-clamp or a quick clamp, what ever is handy to clamp the two sides of the 2x4 together thereby clamping and holding the pen barrel.
This is nice to have in your pen repair kit. It keeps the barrel vertical if you want to drill out a bad component or just use it to hold the blank while you use a punch to knock out a bad component. The soft nature of the pine in most cases does no harm to the finish on your barrel. I have had the barrel slip in this style of clamp with no noticeable damage to the finish.
