The issue of requiring additional filtering in extremely dusty, or very wet/icy conditions was identified on the Mustang III by the RAF in the UK. First became a notable issue when the RAF Mustang IIIs started operating from ALGs in France after D-Day where they encountered extremely dusty conditions which like happened with a number of other RAF types 'overloaded' the filter element, requiring more frequent replacement or cleaning. The other issue was some of the Mustang IIIs flying escort at lower altitudes escorting the Coastal Command strike wing Beaufighters and Mosquitos, where they entered into icing rain with certain areas of the airframe tending to catch and retain ice build-up longer - the NAA multi-hole perforated plate was one such area. The modification was a louvred plate that fitted over the NAA original multi-hole plate which would deflect heavier particles such as dust and icing rain past the filter intake area. By the time the solution was trialled, approved, documented and entered the supply chain, it coincided with the introduction of the Mustang IV, so as they were being prepared in the MUs and by contractors, before initial issue to RAF Squadrons they were the first to get them as a 'mandatory' modification. As Mustang IIIs cycled back thru GSUs, RSUs or MUs, then they would also receive them - this is why most of the photos showing Mustang IIIs with the louvred plate are usually from early 1945 onwards.
