Those grips started life as standard Smith J frame targets that I reworked.
Bottom center in photo:
On a Sg butt J frame the back of my modified targets look exactly like Magna grips on a square butt frame. The bottom has a small gap at the front 'toe' of the grip frame that can only be seen from the bottom.
When I make them for a Rd butt J or I frame (both shorter than the sq bt), I use the excess wood that was removed from bottom and back to make a filler piece for the bottom and the back of the frame that goes half way up.
1. I trace a sq bt frame bottom and lower back on to both sides of the target grips then cut off the two 'L' shaped pieces with a coping saw on the outside of the line to leave a little for final sanding. I sand down the curved or outside side of each L shaped piece to the grip frame width.
2. (For Sg Bt gun, skip to step 3.) Now you have two filler pieces. Glue one 'L' shaped filler inside one side of the target grip while mounted on a Rd Bt gun and install the other to clamp together until dry. Sand to desired shape.
3. Now comes the more difficult part if you want to retain the factory checkering as I did. With a flat file or sanding block, sand off the checkering edges where they were cut thru with the coping saw to match the borders that were not disturbed. Then with a matching width bordering file, continue the original factory border around the checkered area that you reshaped with file.
4. Finish up with steel wool and coat with Birchwood Casey True Oil.