What version of the mt32 do you have? I know there are a couple different ones. I'm not sure if the volume knob (or any other part of the front panel) is different.

I believe mine is the older\first version. I don't have anything super accurate to measure with but mine is 20mm diameter (with a small raised line that wraps around the edge for an indicator) and 10 or 11 mm deep. The half-circle shaft that protrudes from the back sticks out past the edge of the knob about 4mm or so. The knob is all black plastic with no paint.

Also, if your mt32 is just like mine, you'll see that it has a notch on the end of the volume pot's shaft. There is a line of plastic inside the knob that fits into this.

Hopefully this is helpful!

EDIT: I think they're mostly identical on the outside but just in case, mine is the old one with no "phones" jack on the back. https://en.m.wikipedia.org/wiki/Roland_MT-32

Thanks, that should help me track something down. Am not sure what version I have, an earlier type I assume as also not headphone jack on the rear.