Perhaps it is Be/Cu alloy. Whatever it is, it's incredibly hard and refuses to be bent other than where it's bent now. It took a lot of effort to scratch it, and the residue was minimal. It could also be some sort of brass alloy. I agree that it should be replaced with plain copper, possibly two or three layers, especially as copper flashing is sitting around in a roll on the workbench.
AM Radio discussion. Directional arrays are FUN!