I'm pretty sure I know where these originate from but will hold off until Kevin gets the pics of his friends car for us. I'm not holding back on you guys just being judicious here. They are not accidentally "X" double stamped I'm sure of that. Kevin, what size engine does your friend believe is in his vette? Try and get the casting date (see what the dates are for the heads and intake are too if possible). Can you also give us some history on the car please. I would be surprised if anyone would find these letters delineated in any books anywhere.