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THese originally didn't have punched out holes for adjuster access. You'll see where the adjuster hole is in the backing plate, a normal oval shaped indent where you'd expect to see a rubber plug. All you have to do is pop the metal piece out with a chisel and hammer, and then install a plug in it when you're done. I know you said you already tried this, but it's just a matter of hitting it hard enough to knock the metal piece out. It will pop out if it's hit hard enough.