The -5 has the MIM parts and you can measure one and drop it in a bag with fifty and they all measure the same, the old forged units will all be different. Mim is more square, easy to slick up action than the forged unit. Now Please don't get me wrong, I am not in way cutting down the older units. They were hand fitted with love and care, so why screw it up to make a match gun . The older units are a piece of art, the newer ones from -5 and up are the proud product of a machine.