That looks like a crudely ground bare spot down the middle of the board, looks a quick fix for some problem with that revision of the board (version 1.57). I got a version 1.8, it has no line ground down the middle..
The ground line down the middle is just a crude make-do solution to provide separate power to the two blades. I'm thinking they did it that way to use the parts available at the time. Mine also has a ground line down the middle (though a different configuration), and it works fine.