The ridge started to appear in late '71, so the ridge is correct for the '72 model year. I have never heard of a Duster trunk lid being unique vs. one from a Demon. Some of the lids had spoiler cut out's and some did not.
DusterKrazy is dead nuts on.... I have a 71, late build with ridge...... I have a 70 duster with no ridge and a 71 Duster with the ridge.......duster and demon used the same deck lid