Thank you guys for your replys and thank you Peter for that pdf, it's awesome! It's interesting to learn that the first 59 Ms didn't have that air filter. In this case though I'm inclined to think that the aircraft in the picture above has had a cowling transplant rather than a modification as the photo predates that technical order (its caption claimed it was taken in August '43 and even if that's wrong the fuselage roundels and tail markings certainly are earlier than October '43).
@TBolt: I too wondered the same thing, that it could be an M although I doubt that it could've been an N-1 (which the RNZAF operated a number of) as by the time the Kiwi Ns arrived in the forward area (Sept or Oct '43 iirc) the markings would've changed (different fuselage roundels and a white tail unit). If the serial is NZ3064 then it is a K-15 according to ADF-serials (ex 42-10327, c/n 21711), so along with what I wrote in the above paragraph I'm leaning towards it being a transplant job as the Kiwis, like the Aussies mentioned by fubar57, had to salvage and recycle as much as they could due to their limited resources.