MD is so common we start it like an answer machine. Note on the 1991 on the date, the line you are seeing isMD that actually reduces the normal size of the devices. The affected area is lower than the normal device height. This happens after the strike and the movement of the die taps on the device, altering that area. It is caused by the machine. On a doubled die the doubling is on the die. So MD can happen on doubled dies the same way. But a common error found often. The mint doesn't worry about it. If a coin is graded with it, it doesn't affect the grade. We are the ones who deal, with it. The mints job is to make the coins.