[deadin]

Thanks Jim, that's pretty close to what I thought.
However, Ezell in his "Handguns of the World" mentions a Mark I** and a Mark II**. He states that both of these were a recognized "conversion" using a Mark IV barrel and a Mark V cylinder. This could explain the first picture although he states that it should be the 6" barrel, not the 4". As these changes were done in 1915 as a wartime expedient there may have been some 4's in there. From his definitions, the second picture is just a standard Mk II, as there were only minor changes from the Mark I*. The most noticeable being a slightly changed hammer. (And the lack of a hump..)