these have some special features that the article discusses ,

the front bit has different sight protector ears than the mkIII -



the forend has the volley dial and pointer and the hand guards are different than the mkIII -



the rear volley sight arm -



early cocking bit -



the rear "watkins speed" sight -




the cutoff and the charger bolt head -




if i missed something im certain there are others more competent than i here that will jump in with info ,