Tim, to quote a comment you made in another thread, MD does not stand for "medium date". It stands for machine or mechanical doubling, a very common form of doubling caused by a loose die. There is no such thing as a "medium date" for 1970-S.
Wouldn't it be more correct to state that "machine doubling" is not really doubling but, due to the shearing or flat smearing of devices, gives the "appearance" of being doubled? Force clay into a mold and then give it a little twist as its being extracted. It's not really doubled, it just looks doubled.
