Looks like a personnel tag. Miners (as well as many other trades) once used tags like this to keep track of field employees in the event there was an accident or disaster, they would know who was working where. Watch the movie October Sky, with Chris Cooper and Jake Gyllenhaal.
Just reinterating what cti4sw...I'm an industrial field engineer ...I do power plants and petroleum chemical construction and we still use tags today...its called BRASS IN and BRASS OUT....if ur there the brass tag with ur name or number will be hung in the spot ur in that way if there is a explosion or accident in the site the brass tags can be used to determine who is there or not (mines use this method too) ....thanks and happy hunting.