'64 D Nickel; RPM Or MD?

Can't tell.
You only show a closeup of one little place.
We need to see more.

Looks like machine damage to the mint mark. On the building around that area you should see the same push of the movement of the dies on other parts in the same direction. On the left side of the mint mark, it is flat shelf like and not rounded on the top of the area in question. It will show the same color on that area. (if it was part of the design, you would see a light to dark transition) The west side of the upright is reduced in width. MD

Here is a side by side that will show just that.

Note the image on the left. Note the area where the MD reduced the device. That area will all be the same color. Where as the normal device and the doubled die, you see the color change from a lighter color to dark. That show roundness in an image. On MD area on the devices, this will all appear the same color. On the nickel you can see that, but to the left of the device you see it instantly dark. The curved part of the taper is now missing and just a single color is present. On a normal device the light will show a light spot at the top and taper in color till dark. (if you use a single light source) Using the light to dark shows us roundness or if it is flat, all the same color on that area. This mint mark (OPs coin) show that the MD distorted most of the mint mark, but at the bottom of that area you see where the device was, until the MD removed part of it. Here is another coin with a large amount of the date removed and at the base of the device, you see where the area stopped being altered.

Where the device is normal the color fades from light to dark. Where there is MD on a device, it just shows one color. That is what I look for on machine damaged coins.