It is a fastener but the other half was a flat piece that goes through the hole & is then rotated 90% to keep it fastened. You can just barely make out the indentations at either end of the slot that help keep it from turning once it's in place. Trying to remember where I've seen them used. I'm thinking tonneau covers on pickup trucks & old sportscars & that type of thing. Also used on boat covers, etc. I believe my dads 56 Jaguar had those. Here is what the other half looks like. The part in the picture is inserted through the hole in the part you found and turned. Also called "quarter turn fasteners"