I've inherited a totally gunked up commercial grinder, but am stumbling at the first hurdle here

I've taken the grind adjustment wheel off, it won't turn at the moment.
Now is like to get the brass ring underneath off

Can't find any service manuals - does anyone know how to accomplish this?

I have a feeling it might be to do with the three triangular notches which you can just see one of in the photo

Any help much appreciated