Moen Volume Control - Is there a special tool for this?

My Moen shower is leaking so I need to replace the cartridge. In order to remove the cartridge (hex nut in foreground), the large round nut in back with two flat sides needs to be removed first. The problem is that I can't get a wrench around this nut. The opening of the escutchon ring is 2-1/8 inch inside diameter while the round nut is 1-3/4 inches. That leaves just 3/16 inch on each side. The nut is also sunk into the wall 1-3/8 inches deep.

Is there a special tool for this or some method of removing the round nut?

Not doubting you. But I have seen a lot of Moen shower valves, and I don't remember seeing this one.

Is this an Exact Temp volume control? The trim ring looks like it. If so, that ring will pop off of the wall, leaving you a bit better access to the cartridge.

I assume it's Moen because I sent the photo to Moen who then identified and sent me the required cartridge. I'm not familiar with Exact Temp but it's part of Moen shower system TS270 in which the volume control and temp control are separate.

Yup, that is a Moen Exact Temp volume control. I have pulled them apart, but only when I was roughing them in, never to service them.

I have a gigantic adjustable wrench that I would use to attempt to take that two sided nut loose, but that doesn't do you much good.

When you say adjustable wrench, do you mean like a Crescent wrench? I've got one of those but there's no way that would fit in there. Taking off the ring will not provide much more room.

Below is a photo of a SPUD WRENCH. It's jaws may be small enough to enter the opening (after removing ring).

An adjustable pipe wrench may also work.

Can you measure size of the nut?