 The gunsight was changed from the N-3B to the N-9 before the introduction in September 1944 of the K-14 or K-14A gyro-computing sight.Apart from these changes, the P-51D and K series retained V-1650-7 engine used in the majority of the P-51B/C series.The addition of the 85 US gallon (322 l) fuselage fuel tank, coupled with the reduction in area of the new rear fuselage, exacerbated the handling problems already experienced with the B/C series when fitted with the tank, and led to the same fillet being added to -B, -C and initial -D-series versions in the field, to be quickly standardized as a normal rear-fuselage airframe component on later production blocks of the -D version.