Roger at the 70 GTX registry has not figured out the missing dash emblem secret yet.
I have a St Louis GTX without the emblem, pretty original unaltered car. Bought the car from a man that owned it since 1974 and it was not there.
My St Louis GTX has it.
Someone once ran a 70 Hemi GTX registry I thought and they asked me whether my car had the Plymouth emblem on the hood and the GTX on the dash. My car has both. I thought the guys name was Mike Adams but I can't remember names worth squat. Had a red automatic with tan interior IIRC.