If it ain't 3/8=0.375" it's probably 9mm=0.351" and it will have to be filed down any way. I actually just start with a piece of 1/2×1/8 flat steel from the hardware store. File on it until the end fits the groove and then cut the peice off. This gives me a handle while I file. If you drill and tap this little piece, you can put a screw in it to make a flat post sight. I use white nylon screws that cut down to great thick woods sights but are a pain to use against paper. Blued steel, brass or shiny steel can be used and the threads hold paint real good. This kind of sight is crudely adjustable. The screw can lock the piece in. Locktight is good too.