Finally I've got a Kaigelu 316 Charcoal. I've got the pen from jewelrymathematics (EBay), It was tracked all way down to me and it was well protected in a big ball of wrapping bubble plastic and arrived before estimation time! I've got the finial form Richard: it is a huge weight improvement!
The original brass finials come off after 4-5 attempts to remove with boiling water, following Richard's instructions. Because of the brass short finial I didn't had too much room for grip, so with bicycle tubes holding the barrel, and another rubber grip around the finial I grab pliers and hold it firmly, wiggling the barrel until the glue give up. I have glue the plastic finial back, giving a better weight balance.
Also I manage to pull out the nib and feed, using the same rubber grip from an old bicycle tube. One hand holding the section, other the nib and feed with care. The feed looks the same to me, but the nibs are different lengths. The #6 nib I have used from Bexley, cannot be pushed all the way back into the Kaigelu's section; when screwing the cap onto the barrel I can hear some scratchy noise, like the nib is touching the inner cap. I give up and kept the original nib, until the nib unit will come off, and probably will follow Photios instruction and get a Bock nib unit. The original nib is quite good, producing a medium to fine line, having a good feedback when writing. I have smoother a bit the nib, using a buffer to polish.
Now some negative points: the cap came with a big chip where the clip is attached. While is not affecting the writing or the visual too much to bother me, the missing bit is there... Also the Kangaroo coin came off, so I had to glue it back. The original converter it wasn't sealing properly, now I'm using a rOtring converter and the pen is filled with Montblanc Royal Blue ink, is a decent performer.
Here are some pictures:
Thank you Richard, Photios, Flounder, Jewelrymaths for instructions.