Yep, HUD = Heads Up Display.
The story as I understand it is that Mantis changed it for his P51 and gave it to the other 78th pilots and it was re-distributed from there. It may be the same for other aircraft as well. I've only seen it in a screenshot. It has more info than a regular HUD but it wouldn't help me as I wouldn't understand what it was telling me anyway, lololol.
