 As the Mk . IVA was modified into its improved Mk . V , the Mk . VI followed suit . But by early 1941 it was decided to make the Mk . VI an entirely new design , to more easily fit in small aircraft . EMI had already been awarded a contract for a dozen prototype units in October 1940 for delivery in February , but these continued changes made this impossible . Nevertheless , they presented a production contract for 1 @,@ 500 units in December . Between December and March , production examples began arriving and displayed an enormous number of problems , which the engineers worked through one @-@ by one . By July the systems were ready for use , and began being installed in the new Defiant Mk . II early in August , but these demonstrated a problem where the system would lock @-@ on to transmissions from other AI aircraft in the area , which resulted in further modifications . It was not until the beginning of December 1941 that these issues were fully solved and the units were cleared for squadron use . 
