Rohm's main line was low end clones of assorted American revolvers until 2010 when they got out of firearms altogether.
An RG 40 is a clone of a Smith revolver. Gunparts shows grips and nothing else for what they call an RG. No number, but it's a .38.
You might be able to fit a Smith M10 thumb piece that gets attached to the cylinder catch with a nut not a screw. Isn't exactly inexpensive though. The thumb piece runs $18.95 each. The nut, blued, is $6.30. Both from Gunparts. Look on their site by Manufacturer under Smith and Wesson 'K' frame revolver.
Innards of Smith revolvers are pretty much identical. This is an exploded drawing of a Smith revolver that should help. (No such thing for Rohm's.)
http://stevespages.com/ipb-sandw-general.html