Screw-cap pens were offered at a very early date, but what we are discussing here is when screw-caps displaced slip-caps -- both in the industry as a whole, and within individual penmakers' offerings. And I think you'll find that the general pattern is that screw-caps did become the default design shortly before levers were adopted. Older slip-cap designs often remained in the lineup for years thereafter, but any new models came out in screw-cap form.
Incidentally, though no examples have been found to date, Sheaffer did apparently make some slip-cap lever-fillers in their first run of single-bar pens.
The real turning point appears to have been around 1912-1914. Note also that Conklin made the switch across their entire line at this very time, discontinuing all slip-cap models.
That is the clarification I was looking for. When levers started in use the screw cap had become the norm shortly before (though they existed many years before).
Roger W.
