I received a rear valance from AMD for a 70-71 challenger and it has no welded brace behind the license plate as my OEM non R/T car has and i have seen on OEM R/T units.
Speaking to a rep from AMD, he stated some cars had the brace and some did not so the left it off.

Does anyone know which cars received it and why?

I don't see how this thing is not going to flop around without it. The 2 bolt on brackets do not seem enough.