Stormy, it's been awhile since I watched that doco on the desert crash test, but I don't think they added anything to that 727- those stairs were standard fit on that model... they just modified it to allow deployment in flight (or was it removed altogether- need to watch again)
The aft stairs on the 727 were never intended to be operable 'in-flight', but pre-D.B. Cooper there was nothing to prevent their deployment in-flight (I recall the DC-9 having a similar setup). After the D.B. Cooper hijacking, a fix was implemented to prevent in-flight deployment. It was super simple - basically a spring loaded weather-vane that would rotate with free-stream airflow and block the door from opening (again, memory says a similar fix was implemented on the DC-9). Naturally this became known as the "D.B. Cooper fix"
.
I've seen the documentary on the 727 desert crash - as best as I could tell all they did was remove the 'D.B. Cooper fix'
.