Printed with white PLA on my Monoprice Select Mini. After a substantial amount of post-print cleanup of the support material, the threaded parts threaded together easily (it helped when I used beeswax to help glide the path).
I ordered a Rosetta JoWo #6, Ruthenium, Broad nib assembly and it too screwed in without problem.
I've written two blog posts about the experience of printing this:
The pen is light in the hand, but writes very nicely all things considered.
Fused Filament PLA