I know they were used on 65-68's, my area of research, I have not gone into the '69 or later (the 4 years alone is a huge amount of info)
They go on the lowest weatherstrip stud of the rear side rail. Between the weatherstrip and frame rail. Low behind the rear 1/4, looks like they were designed to deflect water forward and not back into the wheel housing area... as there is a drain right below the 1/4 window (at least on 68s)
No picture, as I don't have one of them to mount...! They are shown in the 65-68 Body FAM's.