1994 D Quarter_confirming MD On Obverse/Reverse

Your first clue is this:
What areas of the devices are affected? Note all those areas are the outside edges of the devices. That is where Machine Doubling happens. If this were a doubled die, then on the older dies we would be enlarged devices, not reduced devices:

Of on this image of the DDR, what do we see? We see spread on the centers of the devices and a hint of the two hub issues during the creation of the dies. I'm not sure when the single squeeze dies started to be noticed. On the 1999 State Quarters we seem them since then. But the question comes up why were the DDR's not seen on certain years? Probably because they were either the older style dies, or the DDR on the single squeeze prevented them to be seen on these pre single squeeze dies. I don't have a micrometer, but a measuring the height of the coins would be interesting. The older dies are higher profiles, than the single squeeze dies. So measuring the older style designs and the new style designs, might give us a time of when this started. I feel that it was progressive. Meaning that they may have used up older die first, before creating the newer ones. You could have one of each type on the coins. So we probably won't find just a certain date, but a slow change over? Just a project someone might take on. The results may surprise us?

Thank you for the thorough response, Coop! I'll have to reread your response a few more times to fully understand, but that's what I really appreciate!