I have worked on a few with it, there is a hole in the rear panel and in the alloy trim panel, a square rear fog lamp is fitted, seen it on 1971 cars but unsure when they stopped fitting it on RHD.
e9coupe.com
My 74 has it under the bumper…unfortunately nearly all pictures I have of RHD E9's are from the front. I'll have a look through though to see if I can find any pics of cars without it under the bumper.