I count 10 threads on the section of the slimmer pen, and 12 (maybe 13) on the section of the green & black ebonite pen. Maybe that's the secret to making the joint non-leaking -- lots of turns on the thread.
(Also, I find it interesting that the slimmer pen has coarser threads. Maybe it's molded, where the other pen has threads that are cut?)
The green ebonite is handmade whereas the plastic one is moulded. The internal diameter of barrel at point of threading of ebonite is 10 mm and that of plastic one is 5 mm
For actual size of the ebonite FP see the pics at: