Hmmm, looks like rubber stripping mounting. Thought the rubber bumper stripping days were in the 70's. It should (as I remember) run from end to end. The indent is where the rubber sets in. The square holes are the bolt that lips into the rubber with a flat-rather large head on the bolt.
You can probally still get those from Macs or some place like that.... Do a Google search for the year/car and see if you see your bumper on those cars.
Ok I'm gonna dig through the bucket of stuff that I have, maybe there in there. When I got the car off my brother it didn't have the motor, trans and other things because he was going to re due it so when I got it I had a big task, that was fine just didn't know what the side holes were