1968 Dollars - Waterline Lumps/Dots

I will start off with what for me were some interesting stats. I got hold of 100 1968 dollars, all in pretty good shape.
Of those 100 coins:

9 were "Small Island"
4 of these had slight rotation
7 had REGINA doubling/die error
7 had what I call "Lumpy Lines"

I was interested in learning more about the waterline lumps. The first one I saw I thought was just post mint damage, but then I found more with the exact same spots/lumps. The photos below are from 4 of these coins, and you can see the lumps are all in the same place.
Has anyone seen this before? I thought maybe it coincided with the doubling of the lines, but that is not evident to me.

These are 4 different coins.