They are NOT all the same! Actually, there are three different designs. Only the 1955-57 will interchange. The 1958-59 grille pods will not fit on a 1955-57 GMC. The shape of their back is not the same because of their different position on the front bumper.
Even the 1955-56 is different from the 1957. The earlier grille pods do not have the center dimple. See Photo. Therefore, to the real perfectionist, ONLY the 1957 has been reproduced.
BEWARE: The current reproduction GMC grille pods are advertised as 1955-59. Not correct for reasons mentioned above! Somehow the person providing a sample to a fabrication shop, must not have known his product would not fit a 1958-59. Too bad!