At the end of June 1944, a decision was taken to fit tropical air filters as standard, similar to those fitted to the three Typhoons which had been sent to North Africa in 1943. One thousand sets of the filters were to be manufactured and fitted to front line Typhoons as Mod. 421. It was estimated that these could be fitted to all Typhoons on the production lines by the end of September. Research shows that late Typhoons starting in the "RB---" series were fitted with the filters, as were some rebuilt aircraft from earlier production batches. Mod. 421 appeared as a streamlined rectangular "hump", just behind the main radiator fairing and between the inner wheel doors, where the updraught carburettor intake was located.