my 150 has the thru prop hub exhaust, two just below lower powerhead cowl as usual. but it also has 3 each side of cavitation plate pointing down of course? none of my other merc have had these 6 ? or any other i see. any idea why i have them, sounds like i`ve got headers on it when coming off plane from high speed. perfect hole shape and looks like factory paint so i don`t think they are homemade.