You're looking at an original German Army issue duffle bag lock. It's a lock cast from plated steel designed for use with military-style duffel bags that feature grommet-lined openings. Simply run the lock through the grommets lining the opening of your duffel bag. The attached clip secures the lock by simply slipping over the eyelet. For enhanced security, a padlock can be used in place of the clip. The pin is 3 1/4" long and 7/16" wide at the end. The whole lock is roughly 4 3/4" by 3" in size. These come in a pack of five.
Note: this is not compatible with the Hungarian duffel bag