L-shaped nail hanger thing
Does it have to match this one exactly. It is a flooring cleat, but I don't know where you might find a heavy one like this one, obviously intended to be hand nailed. They still make cleats with this shape, but they are much thinner in profile, as they are now driven out of a flooring nailer tool, which loads them in collated racks, like a nail gun. If you can use the thinner ones, I'd try to beg a rack or two from a local floor man. If you have to buy them, there are going to be several thousand in a box.