Yeh I have had a few tr3s myself and none of them had a bracket. I did have the seat keep sliding out of those slots one on and drilled a couple of holes on the very lip of the back side of the seat and pulled the seat in from the trunk and wired it to those C shaped brackets that the seat wedges into.
I have no life, so finding new things on a tr3 interest me; how exciting. Anyways, when I noticed how the tub had a recess where the back seat goes and there was a corresponding hole in the seat, I thought I had figured something new and astonishing out. Perhaps I could make them and become rich.
The way I see it there was some engineering for something there and maybe the early cars had something.
David posted a bracket for the steering column that I had never seen before-- plus I kinda thought before there should be something there to hold that side. Moreover, the radiator over flow tube bracket was unknown to me until a couple years ago, so there is hope for more unknown parts on a tr3 kind like "exhibition unknown" on TV.