The War Office examined the design and put the prototype through a series of trials during May and June 1938 ; the model was tested as a possible " light cruiser " since War Office light tank needs were already met by its predecessor , the Mark VI . The War Office then took the view that the tank was not acceptable as a light cruiser because the Nuffield A13 offered better speed and obstacle crossing performance . Despite this , it was decided that it was essential for some Tetrarchs to be produced , and it was suggested that they be brought in at the end of the light tank program . Accordingly , the War Office gave the Tetrarch the official General Staff specification number A17 , and , in November 1938 , accepted it for limited production after requesting a few minor changes which included the fitting of an external fuel tank to increase the tank 's range .