Just to clarify, I don't think the "part of another card showing" aspect is entirely correct. For instance, this is my 1969 WL Mantle from the March pickups thread. There isn't another card specifically showing (of course, some contrarian will argue that the pink on the left side is most certainly "a part of another card"), but a bit of Mick's name may be sheared off, and the number bubble is definitely cut. So this type of instance can be added to the list of things leading to an "MC" designation...
1969MantleWL500frontback.jpg