Conklin Metal Pens - 1918-1926 - These pens differ from the overlay pens in that they do not have a hard rubber core.   The section simply attaches to the all metal pen body.  The earliest models have the cap threads on the nib end of the section. This proved to be a bad design since pulling on the cap pulled the section out of the pen. Later models have the cap threads on the end of the metal pen barrel. The section fits into the barrel with a slight twist.

These are found in longer versions that have pocket clips and similar pens that are lever fillers. All of the pens that I have are fitted with #2 Conklin nibs. I have not seen any larger models but they may exist.

The caps of these pens are all metal with a hard rubber inner cap. Ink and liquids get between the cap and the inner cap  and corrode the metal of the cap. The damage can be seen as bubbling on the surface metal in the middle of the cap. This is not a repairable problem so be careful when you buy these pens.