 Continued work displayed a number of minor problems , and the decision was made to introduce a redesigned unit with significant improvements in packaging , insulation , and other practical changes . This would become the AI Mk . V , which began to arrive from Pye in late February and immediately demonstrated a host of problems . By this time the microwave units were being designed , and the Mk . V was almost cancelled . A contract for over 1 @,@ 000 units was allowed to continue in case of delays in the new units . By May the issues with the Pye design were ironed out , and the FIU 's testing revealed it to be superior to the Mk . IV , especially in terms of maintenance . An RAE report agreed . 
