I owned a 67 RSSS 350 car, muncie, 12 bolt 3.31 posi, L.A. car from 1981-1994 with no radius rod, car's build date unknown.
I also owned a 67 firebird 326, muncie, 10 bolt 3.55 posi from 1981-1982 and that one had two rectangular radius rods, one on each axle tube. I believe that was also a L.A. car, build date unknown