Just behind the front spring eye, mount.
I have nothing to base this on but the HIGH end restorations I've seen at MC&CN.
I looked for the tag residue on my 68 and only found remnants of the front spring tags still on the car, rears were long gone.
Neat detail item.
However (I know you did not ask) I Did find the yellow w/black block letter "PP" tag [open 3:08] that was (Still) attached to the passenger side drum beaten and battered but still intact wheel the wheel mounts.