1980-D May Be Double On Reverse

That is all machine damage after the strike on the coin. How do I tell? Note on 'OF'? You can see on the top of the 'O' that the devices were reduced and not enlarged. (as well as the rest of the devices that are altered after the coin was struck)

That means the die that struck your coin jumped a little bit and struck it again. The "jump strike" was offset a little, and flattened parts of some of the design. The parts that were flattened resemble doubling somewhat. Since the flattened parts came from the original devices, the original devices are left reduced in size. This is referred to as Strike Doubling or MD. In general, it lends no extra value or collector interest.
Another form of MD is wear on the die. It too, reduces the devices and resembles doubling somewhat. What you really want to find is doubling that doesn't reduce the devices, a Doubled Die coin. On this coin, you will see a normal sized device, with doubling also, or a larger device with no apparent doubling. If you are ever in doubt what the normal sized device should be, you can usually compare your coin in question with another coin of the same denomination, year, and mint.
If you read what I have written for you enough times to fully understand what I have said, you will be well on your way in identifying the most sought after Variety coin of all, the coveted Doubled Die.

Gotcha Coop, thanks for the correction. I don't think I was too far off, it struck the coin normally and then moved again flattening parts of some devices all in the same strike. The bounce or "jump" actually struck the coin for the second time, all in the same mint strike. Once to make the device and once to flatten it. Correct?

That would make a fantastic video.