Like Uncle Bob said, VERY common item in the garage. You could speculate all day as to what it held for the P.O. and probably be right on every account.
Mark
Saw one on a toolbox that was used to hold the end of an extension cord. Toolbox was in front of the outlet. I'm sure the correct answer is to use it for whatever you find convenient.
Those clips come in many sizes. At a prior job I ran into many of them used to secure the end of a short cord to keep it from dragging on the ground.
One sized for a 120v power cord would hold a 3/8" extension nicely.