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.
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