The M-1944 packboard was designed as a "one size fits all" hauling system for bulky items like ammunition crates, radios (such as the SCR-300), mortar parts, and gas cans.
Lashing Loads: The 27-foot rope was threaded through the eight metal hooks on the sides of the packboard to "lace up" the gear, much like a giant shoelace.
Lacing the Canvas: Smaller lengths of similar rope (often ~76 inches) were also used to pull the canvas back-panel tight against the frame, creating a comfortable "suspension" system for the soldier’s back.
Material: (I believe) is braided cotton.