I was beginning to wonder if I'd ever see one... The crossbar of the "G" is fairly faint, but it's definitely there. It appears the "G" was lightly struck, and then overstruck with a "C". This is not reflected in the [120] price guide listing, however.

I've since given this insulator a bath in oxalic acid, but the error shows up much better on camera with the soot still on there!