Paul pondered...
My guess is it's an Americanism for a lock nut, a thin nut to go under the head of the screw and against the EBB shell.
If so that's a pretty good idea, does it need a thin spanner to tighten it?
...and Matt commented...
Ah yes Your suggestion makes sense.
We'll wait for Dan to surface in USA.😉
...and Dan replies (wondering how this question was missed! I usually get them all and right away...)
Yes! Paul has it. These are simply thinner nuts with the same threads that spin on the shaft of the machine/grub screws that hold the eccentric in place. Because the eccentric screw threads are so fine (M8 x 0.75...same as chainring bolts), the jam nuts don't have to be tightened very much to lock things in place -- just a bit more than finger tight will do it. Mine happen to be 3mm thick and 12mm across the flats, so they do require a wrench specific to them or (shudder!) pliers (as the torque needed is so very low). I use my needlenose Vise-Grips (think: MoleGrips) for the task and have no problems with scarring or rounding, again because the torque needed to lock everything in place is so little.
They do work a treat and I made sure mine are stainless so there's no problems with rust. I have installed them with some very thin stainless flat washers between them and the stainless bosses on my Nomad, only because I didn't want to ruck up or mar the powdercoat on the BB shell posts.
Hope this helps, apologies for the late reply, Dan.