It's just a flat metal piece that's stamped out. I'm sure that I won't remember it, but I'll try to take a picture when I'm at my old shop again.

Unfortunately, there's almost as many flat, stamped-out wrenches in his tools as drill bits. :grin: :hmmm:

Dang, I was envisioning a bucking tool for real rivets.

I found mine. I thought it was the same as yours, but it's a POP Rivet brand. It looks a lot like yours. Here it the wrench that came with it.