The guards on my truck were NOS in the factory sealed box when I purchased them in 1992. One seems to fit better on one side than the other, but they are both identical.
There are actually two different factory installation sheets for the bumper guards. One shows them mounted over the factory bumper bolts, and the other shows them exactly 3” outboard of the bumper bolts.
This is the instruction sheet that came with the guards I purchased. I don’t know which template GM produced first:
And these photos show how tight they fit against my original bumper:
As far as I know, the only difference between the 60-62 and 63-66 bumper guards is in the vertical spacing of the bolt holes, since the later frames are taller.