It's a matter of material and/or finish in the first five Marks, and specific features in the other two. The Mk. I was matte blued carbon steel, the Mk. II was satin nickel, the Mk. III hard chromed, the Mk. IV polished blue, and the Mk. V was matte stainless. The Mk. VI was a deluxe version of the Mk. V, with adjustable rear sight and polished finish. The Mk. VII was a special hideout or deep concealment version with no sights, also in stainless.