The 60-62 frame is shorter in vertical height. This spaces the inboard vertical mounting holes closer together than on a 63-66 front bumper. If you have a 60-62 frame that originally came with torsion bar suspension, and the bumper bolts on without any extra holes or modification, you have a 60-62 bumper.
Fyi - here is the factory template for mounting bumper guards on a 60-62. The vertical dimension between the bumper bolts (not shown on the template) is 3.25”, measured directly off the master. I don’t know the dimension for a 63-66.