This part has a pocket of deeply embedded mill scale just right of center. It will need to be “pecked” by hitting it with the corner of a chisel in short and fast strokes.

Next photo shows hitting with a chisel, no hammer.

23-9 Bumper Support, front.deep mill scale, possibly rolled in..jpg23-9 Bumper Support, front.pecking deep mill scale with a hand held chisel.jpg
If you don't see two dolphins, you need a vacation.