Question about rear bumper and leaf springs for a 55 1st?

[LeftEar55]

I would like to know if anyone knows why the 55 1st series rear leaf springs are so outrageous? How and why, are they different from a 47-53? Also, been looking through several catalogs and have noticed that the rear bumpers are different for the 54 and 55 1st, then the 47-53 models? How different, exactly, are they? I saw somewhere that the one of the different bumpers has the center section notched out for a license plate. I do not want mine like this. I want mine mounted under the lefthand taillight. Can someone help me out here with these 2 questions? Thanks