The marks are Japaneseicon. I have a Kokura made type 99 and it is covered with that same mark that resembles a 4. I am assuming, and I know that is dangerous, that it is the symbol Kokura used to stamp their small parts. Either that or they were subbed out to another contractor. Usually there is only one per piece of metal but I have a rear sight sitting here by the computer that has 3 "W"s on it so may be a common thing to stamp the sight multiple times.