Exactly. The '74 Challenger set that I put in my '68 Charger had heat shields and had the 4 holes where this one has 3. Your '73 Challenger has the 4th hole. If you look at post # 3, that set is ID'd as 70/71 E-body 318, and is identical, including missing the front lower hole, to the '69 B-body set I am trying to use now, as seen in post #1.
Did we discover a type/application of mount that doesn't work? My mounts are from a '69 318 B-body and look the same as the 70/71 E-body, and both don't have the 4th hole.
