I remember reading on the safety card, I think it was on Eastern or Northwest, that some of their DC-9s had the rear stairs under the tail, and some didn't. Those that didn't required you to pop off the tail cone if you had to use it as an emergency exit. I don't remember the rope, but there might have been one. You can tell which type of DC-9 you're on by looking at the rear of the cabin. If there is a full-sized door, there is a stairway exit under the tail. If there is not a full-sized door, but a small hatch about half the size of a door, then you don't have a stairway exit, but you have the kind where you have to release the tail cone.
I really think I read this on Northwest, since they got their DC-9s from so many different airlines, which would explain why they have both types.
Fly Eastern's Golden Falcon DC-7B