70-71 Dart and 71 Demon fenders are the same. 72 Dart and Demon fenders are the same.
70-71 has a flush mounted side markes and 72 had the block style. That is the only difference in the two. They will interchange with each other but the marker lights will be different.
'70-'71 Dart Fender will fit but has different type of Side Marker Light.
They can be cut and changed if you are skilled enough.
So in reality, only a '72 Dart/Demon Fender will work.