I've seen owners take off the pinion mounted one (the bracket and bumper) and place a bumper in the upper/floor mount on their K code just because they were uncertain and because their buddies car or one in a magazine article had one mounted there. Asked one one time why and their response was that if Ford didn't want you to mount a bumper there they wouldn't have put holes in the bracket to mount that bumper
Just offering another view on an "out of the ordinary" finding
I agree with what you're saying about people copying what they see in pictures but for at least my example, I have only recently cleaned it after 35+ years and knowing the original owner attempted zero restoration, I can safely rule that out. Could have been installed during a service trip in the 60's or 70's but I don't see why anyone would bother. Another guess is that assembly line workers were so accustomed to mounting these to 8" plates that when a 9" rolled down the line and they could do what they were doing most of the time, they would out of habit bolt one on....?