I've got photos of a supposedly untouched 71 Duster with the flat spot on the bottom.

I've got photos of a 67 Dart GT I had that was basically virgin other than engine mods, and the flat spot is toward the inside.

I've got photos of a 72 Duster I had that I am pretty sure had never had the heater box out of it, and the flat spot is toward the inside.

It gets better - the illustration in the 1971 Chassis Service Manual doesn't show a flat spot - it's round.