Very beautiful pen, and I am always impressed by the architecture of the Jr Abraham.
But ... I just don't know the answer to your question, Steve.
I have the feeling ... and it's only a feeling ... that discriminating pen USERS are either in one world or the other ... fountain pen OR other pen. . Perhaps there are discriminating COLLECTORS who do want the "best of both worlds" ... just don't know. . Do discriminating COLLECTORS actually use their pens ?
Speaking for myself ... if I want to write with a fountain pen I want it to be available as a fountain pen ... I don't want to switch it from a rollerball to a fountain pen, and I most certainly don't want to do the reverse switch ... because how/where do I then store the fountain pen nib and the attached, partially full ink cartridge.
I can see having two functional pens ... one of each ... but I can't relate to changing parts from one to the other.
Having said that, I do recall having purchased a couple of convertible pen kits which I have not yet made !