I guess I just invented this mod, I called it the Energel KT. It’s using Pentel Energel barrels as caps on each extremity.
21 centimeters (can be longer or smaller)
18.5 grams (can be heavier)
It uses a real and original miffy&melanie from the Brand Kutsuwa (not from chinese fake from psershop)

I’ve used these >Pentel Energel< found this week on a penstore. It exists all these versions :

From http://www.ebay.ca/itm/Pentel-Energel-0-5-mm-special-pattern-barrel-Blue-ink-6-pcs-Set-B-/322204558426

I like so much these pens ! <3