Correct: The opposite flat is stamped "Bull's Eye". My very unscientific study of these plugs tells me that there are many varieties, which is interesting since it is a pretty scarce plug. I am aware of the one shown, one with the bull's head (I'm told there are two versions of that) and a mica insulator. I was unaware of one with with block letters that you describe. It's funny how many Connecticut plugs have locations on them. Hartford, Breech-Block, Hall (hampton and Willimantic), Daly, Norwalk, Fitzco and Ideal Switch all have a plave name on them.