I'm not sure, but I think those are the mounts for a normal Sierra bumper and would be plastic-welded directly to the inside of the bumper-shell.

If I'm right, I don't see how you can use them on a Cosworth unless you can find a way to glue them to the inside of your bumper because if you rivet them to the remains of the real mounts, surely the bumper will stick out too far?

This is the correct panel, but hard to find and a really shit, fragile design: