undercoating inside the doors and sound deadener

So, my 68 Dart has factory undercoating inside the doors.(on the inside of the outer skin) There are no impact beams or anything inside the doors...they are just a hollow shell. Did the factory add an impact beam inside the doors at some point? What year? If/when an impact beam was added, did they omit the undercoating in the door?

I'm in the process of applying butyl sound deadening to the car. The floor is done and I'm to the point of doing the doors. Most of the threads I've read, people say to apply the sound deadener to the inside of the doors. They don't say anything about undercoating being there already. I'm questioning whether or not I need to add it to the doors if there is undercoating there since it's a sound deadener in itself. Then there's the fact that I'm not sure how well the sound deadener would stick to the rough texture of the undercoating.

Maybe the later years have an impact beam and no undercoating...thus the need to apply sound deadener in those cars? Thanks