1965 Dodge Dart Charger convertible

Jim! I totally agree with you. But after hours of searching and thinking, the only thing I can come up with is that they used power tools.
They hanged the complete system and then started to tighten the bolts from the back and moved forward. Before that they mounted the rear rubber bracket and the rear steel bracket above the rear end.
The notch needed to be so much open to accomondate a power tool to bolt the resonator to the rubber bracket.
That is the only thing I can come up with.

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You may be right, Ulf but the hanger is the same on the '66 models without the notch and there is a bumper in the way, although the exhaust may have been attached before the bumper. I prefer to think there was a miscommunication between the engineering and design teams. It would have looked much better tucked up into the notch in the rear valance.
When I had my 1st '65 Dart back in 1966 people would ask me why my exhaust tip hung too low. I asked the dealership and they told me that's the way they all came and it was supposed to hang down like that. I just accepted it but didn't think it looked right then and it still doesn't to me, especially when you look at how nicely the Mustang GT exhaust trumpets of the same era fit through their rear valance.

I guess we'll never know... :)