Just a thought. In many places of industry, tools are supplied to employees through a central tool room or warehouse. I have worked in places that are like that. They assign a number to a tool and when you check it out for use, you get a tag with the tool to identify that you are the one that checked the tool out and to prove that a tool is yours if you find someone else with it. When you return the tool, you give the tool and tag back. So perhaps it was the id tag for a rivet gun of some sort.
