This is an addendum to the post about the Selfit nib. Andy R. was kind enough to obtain patents for me and they made explicit the purpose of this nib. The three “bumps” on top of the nib, which I barely noticed, act as threads. Doubtless the barrel of the Selfit pen was fully threaded, making this a screw-in nib, perhaps the earliest. Esterbrook, Platignum and Osmiroid nibs came much later.
A later patent indicates that the first screw-in nib may not have been entirely satisfactory and the threading on the nib was improved. The rarity of the Selfit pen makes it clear that it did not take off but it was a pointer to the future.
Wyverns’ thinking behind this development was that removing a normal nib from its section for cleaning or replacement was difficult and could lead to breakage of the feed or barrel. Pen manufacturers continued to make friction-fit nibs for decades after the introduction of the Selfit and pen repairers did not report any great difficulty. When screw-in nibs did come along, it was to enable ease of changing nibs for schoolchildren and calligraphers.