Very difficult to do. You probably won't be able to reuse the grommet once you remove it. It will take a special.punch and some expertise to reinstall it. There are a lot of liitle parts in the forearm cowl.
If you can avoid removing it, you're a lot better off.
Otherwise, get the 391 manual written by Seamus O'Casaides sold in the classified section on Shotgunworld.com. You should be able to buy the grommet from Coles gunsmithing.
It is really nothing more than big rivet that needs removed. It costs like 3.00 and is not difficult to re-install but a piece of brass works best on outer side to keep from marking the rivet up. I have done a bunch of them when having the wood refinished. A dremmel tool will remove the original rivet from the inside of the forearm.