Might be of Interest that the NA-97 A-36 had several armament proposals including four 37mm (similar to he two 40mm 'S" cannon on AM-106). The 20mm version was actually flown on AM118 during the modification cycle to test dive brakes and bomb/fuel tank racks.
I have a theory that the slanted gun mounts that plagued the P-51B were left intact to accomodate future 20mm installations as the models morhed. For certain the Aussies wanted a 1x20 1x50 cal wing on the NA-110 Mustang contract. AAF said 'no'.
