"Was a small rubber grommet, pretty sure it was a stop for the back of the seat."
"That bracket does NOT show up on 80-83 LHD FJ45 gassers.."
Hmm, okay, I can see that it might have been a stop for the seat back. The cab upper section is after all off a RHD truck. Yet another little difference between LHD and RHD if that's the case. I may have to move it over in behind the driver's seat then.
I fired off the pictures to someone with the EPC data at their fingertips to see what they can find, and if a rubber grommet goes there, hopefully it can still be sourced. My seat is going to be all the way back as it is, and I wonder if I might just as well axe the whole idea of using the bracket, as the last thing I want is another part restricting the space, even if just a tiny bit. Also, I imagine that if it is a seat back stop, then that would work best on seats with a metal backing plate?