Not long ago I acquired a nice Wirt dropper-filler -- a classic slender black chased hard rubber overfeed straight-cap, with 1882 and 1885 patent dates on the barrel.
Yet there was something more lying in wait. Look at the picture at top and the picture below, and you'll see there's a little step at the very mouth of the section. And the section doesn't seem to have a threaded joint separating it from the barrel . . . .
With a careful application of water (just at the section mouth, avoiding spotting of the hard rubber elsewhere), a bit of patience, and some careful wiggling, the nib unit came out as shown below.
A "jointless" design, very close to Parker's, patented in 1899 (US patent 622,256). I don't know of any advertisements for this particular Wirt model, nor is it familiar to the Parker experts I've been able to consult. Perhaps Wirt licensed the design, though it could also ave been a case of trying it out unofficially to see if it was worth licensing.