This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
The illustrations show two methods of securing papers or photographs in a mailing tube. In Fig. 1 the mailing tube A is shown in cross section and the manner of running the string through the pasteboard walls illustrated. The ends of the string are drawn up and tied over or under the label. If the label is pasted over the string or string ends as they are tied, it makes a sealed package which is rated as first-class matter.
The second way is to run the cords in a cross form through holes near the ends, as in Fig. 2, then tie the knots as at BB. In each instance, the String in Either Case Prevents the Papers from Slipping Out of the Mailing Tube even if the papers fit the tube loosely, they will be held securely and can be easily extracted when the knots are untied.