If you go to my website http://www.martinihenry.org/ and click on "the weapon", I have every single mark of Martini, from the 1st trial pattern through MkI, II, III, IV,V,VI, .402 experimental and .303 variants.
In terms of the the blocks, the MkIII block introduced in 1879 was designed to be be interchangeable, however it was the subsequent adoption of the much stronger extractor in 1883/4 that required the front edge to be chamfered hence the adoption of the MkIV block (actually designed for the cancelled Enfield Martini project).