Some one line imprint pens have complete unmarked nibs (still unvented of course). Some appear to be a flimsy cheap spade shaped nib, but these have tipping material, so I suspect that they are period correct nibs.
Most Wirt nibs have a size number. Catalogs often only mention sizes 1-7. I am not aware of anything larger than an 8. I acquired the #8 nib later, so it is not in the large nib photo. However, it is shown next to the 7 from this picture as well as next to a typical standard #2, and a “nice sized” #4.
Very rarely seen is the Wirt crescent nib.
Some later pens (but pre 1920s celluloid pens) have the Wirt “wreath” nib.
Only the late celluloid pens can be seen with vented nibs and traditional finned feeds.
Wirt nibs #1 through #7.
Wirt nibs size 2, 4, and 8 
Wirt nibs from the US Senate pens, size 4, 7, and 8
the “Wirt wreath” nib came in different sizes, but no size marking 
plain ordinary nib seen on very late plain ordinary pens
Wirt crescent nib
This is a very unusual nib. I had one on the Wirt Jointless pen, but it was unmarked. I had always wondered if it was truly “correct” or just correct to the time. Then I found a standard two line imprint Wirt pen with paddle overfeed. This crescent nib has a WIRT imprint. Side by side, I believe the nib in the Jointless Wirt is correct.
