A 6 mph difference is statistically irrelevent. You could fly two identical models and one could be 6 mph faster than the other on any given day. 6 mph out of 420 mph is 1.4 percent.
They are published test speeds from somewhere, and just as easily the J was flown with a slighlty better engine that day, or a polished leading edge, or who knows what, maybe the bird crap on the nose wasnt polished off properly.
Any diff under 10 mph isnt even worth discussing really, despite the endless threads on plane X and plane Y.
My guess would be either a P-38J-25 or a P-38L-5 could do @ 410-420 mph tops in good shape, clean, with a pilot that made no mistakes.
