Door Panel Question

Hello!

I want to purchase a certain part for my vehicle but can't seem to figure out what the name of it might be. It's the part that sits above the Driver side Door Panel or Passenger, it has a hole for the Door Knob.

My car is a 1974 Plymouth Duster and you can see a part of the "Sill" in the picture that I have posted. Mine is broken in half and I am just trying to find another or find where I can purchase something like that so if anyone has any idea of where I can purchase it and also what It's exact name is, that would be much appreciated.

Thanks for the help guys, these forums are extremely helpful and very noob friendly.