They were supposed to be on every Six Pack, Hemi, Vert and I believe 440 cars. I have seen them on my later 440-4bbl B-bodies.

As for why they were occasionally missed all I can figure is that the torque boxes were welded before the paint. Maybe the body got seperated from it's paperwork the bodies got out of sequence. So that /6 ended up with a Hemi body and vice versa. That is purely speculation though.