Looks like Tuff Line/Cortlandt or Gudebrod, Greenspot, Braided Dacron, I have spools of it for fishing. It's very low stretch, some claim 2%.
I'm not sure of the gauge, could be 130 lb test, I grew up using it.
It's also hollow which allows you to splice loops, and single monofilament line leaders. Basically like chinese fingers, it becomes one continuous line, no knots.
I wrapped a Frank Paradise Titleist with 36 lb green spot, it was horrible.