Hm. I can't find anything about those.
One thing I did find was that the Picts were also described with similar counterweights at the ends of their spears. And that Mciann's drawings were romanticized.
Be careful googling Pict spears though. The Picts were said to fight naked and you're liable to get an eyeful.
But here's a Pict spear with the same bulb.
A similar, but longer, spear can be found here:
https://allthatsinteresting.com/picts The description reads: "A depiction of a Pict warrior, painted as described in Roman history."