I don't know about the first but the second and third gen. plugs I have here. The second gen has a shallow (1/8in or less deep) concave about 1/4in diameter in the center with a flat shoulder(not avery good design, just my opinion). The third differs by having a deep ( 3/8in roughly) concave with a smaller shoulder. It seems to be more accurate in my gun. I never would have thought that a breech plug would make a difference, but it did.