Very different than what you imagine.
The section unscrews, but you will have to use heat - and be careful. The celluloid softens and distorts easily because the barrel is relatively thin.
The filler unit unscrews, but it uses a regular sac inside. You need a special spanner to unscrew the nut in the front half while holding the nipple that attaches to the sac, which you have to then coax out of the wall in the middle of the barrel. Don't lose the piece that holds the sac inside the knob assembly. Assemblign is tricky too because you have to get the sac just the right length. It's all fussy and fiddly.
Come to think of it, if you like the pen, you might want to think about sending this one out to an experienced repair person. These are a challenge even if you've been repairing pens for quite a while... especially if you have to make pieces that are missing or broken.