Instructions
Open the Army duffel bag's single top opening, either by loosening its rope in the top hem's brass eyelet (if manufactured before World War II), or by unlatching the hook from the top grommet and gathered eyelets (WWII and beyond).
Look along the hem of the opening for stenciling.
Write down the numbers you find for the bag's serial number, specifications and any identifying markings for the lot number.
Look along the body of the duffel bag for more stenciling, larger in size.
Write down the name of the bag's most recent owner and that soldier's Social Security number, referred to in some military circles as that person's "serial" number.