I believe those are stock on 58-59 but not on the earlier ones. My 58 6400 has them, but not my 55. The fenders are wider on the 58.
Running Boards are stock on all 1956-59 5000/50, 7000/70, 9000/90 and 10000/100 Series Chevrolet trucks.
They were optional (RPO 423) on the 1956-59 4000/40, 6000/60 and 8000/80 Series.
All of the TF 2-1/2 ton Chevrolet trucks had the extra wide fender flares...
Mike B
