They are not hard to make if you have a lathe.

I have done a lot of them. You simply cut an elongated "tear drop" and then file or mill away the top so it has the shape you like. The common ones are just like those on the Mausers in the pictures, but the other style has a ridge in the center. Look at this picture of one I made to see how the ridged ones look. this was done on a Military Breda Mannlicher so it has a bit more flair then some on the original Mausers, but the way it's made is the same.