centers are a lot easier to find then the ends or the add on step. then again who can find the center for a packard. that's not a cheap item either
Keep in mind that they did not all come with the bumper step. It was an option on M-M hearses and combinations. You would not necessarily have to put one on it.
no step on a hearse. it was a option. they use the normal slotted licence plate holder with it drilled and bolted in the bumper. you can make that 55-56 center work try it let us know how you made out. that the only way you will know. I don't have one here tat I can compare the 58 one with. if you have the bumper end see if they mate up.