The 5" round mirror (with black painted back) was standard on Advance-Design trucks.
A 5x7(?) rectangular mirror was apparently only available as part of the extended bracket (adjustable arm) RPOs (RPOs 210H & 210J).
Of course, dealers and owners most likely put larger mirrors on the A-D trucks. The trucks that I bought in the late 60s and early 70s all had 5x7 replacement mirrors (with white painted backs) on the short mirror arms.
This is most likely a case where originality, safety, and convenience (and appearance) are in conflict.
A convex mirror does help on the passenger side; and, 6" round flat-glass and convex-glass, black-back and stainless-back mirrors used to be available at truck dealers.
