The only documentation we have is when a customer calls for an exhaust system and claims the 70 manifolds were on the 71 car when bought new.
Also probably for the same reason that 1971 models are supposed to have only one exhaust hanger floor pan bracket(left side) but some have two like 1970 models.Learned a long time ago "never say never" with Mopar. Same issue with some early 1970 B-body cars having 1969 exhaust manifolds. An old Chrysler assembly worker told me one time that he remembers using up previous years inventory before installing the new inventory. Makes good business sense but who really knows. Documentation is great most of the time but in the real world sometimes is not reality.