I think there's still reason to be skeptical. Anything a 19thc writer puts down a half century after the event is bound to be wonky!

As to the 5-line Highland Laddie:
Its pattern resembles "Billy Boy," another northeast England or Scots song that lightly appears in some literature about "chanteys."

The question, again, is whether screwmen actually sang that or if (as you suggest) Erskine tweaked it.

Unfortunately, I can't honestly rule out the former possibility. Whereas, I believe, the method of hauling topsail halyards virtual compels halyard chanties to standardize in 4 lines, cotton screwing work does not compel form to that degree.

My theory has been that screwmen of this era pushed/pulled, *without singing*, after each chorus. They didn't require a certain number of metrical beats (or a steady pulse). Erskine's 5 lines could "work" just as well.

It does break the pattern of what I've seen, but the sample size of "cotton screwing songs" (which is creating that "pattern") is small.

By the turn of the century, screwmen had evidently completely changed to singing different styles of song. In Natalie Burton's 1918 collection, the screwmen's song recently remembered by James Scott of Savannah took the form of an African American "hammering song" (my interpretation), e.g.

Screw di cott’n,
heh!
Screw di cott’n,
heh!
Screw di cott’n,
heh!
Screw it tight—
heh!

Screw di cott’n,
heh!
Screw di cott’n,
heh!
Screw di cott’n,
heh!
Wid all yo’ might—
Heh!

Then there's this...
https://youtu.be/bfXK47rT1jI?si=5lTfIL7SDYXPcYXM