Spools of packthread.
packthread definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
strong, thick thread or twine for tying bundles, packages, etc.
packthread definition by American Heritage Dictionary
A strong two-ply or three-ply twine for sewing or tying packages or bundles.