Found this while I'm STILL going through the 75 pounds of copper pennies I have. It seems like hub doubling but I can't find any mentions of it on Variety Vista or any other websites. Thanks in advance!
Actually MD is caused by the die moving just a little in the microsecond after the strike as it rises, because the die is a little loose in the holder. The looser the die, the more shelf-like shadow. Once the die is retightened, everything is back to normal with crisp strikes