I have not paid much attention in the past couple years, but...On page 12, 43e is rear bumper - carbon black with integral step, optional on low roof only.
It doesn't have a charge for it, but it is optional and so doesn't come standard. Every Transit we have seen has a "step" sticking out that's level with the trunk. Is 43E an additional step?
On the full-sized Transits, the low roof had always had a bumper that is a couple inches deeper (more room to step) than the medium and high roof Transits. As to why was a matter of conjecture. MY conjecture was that a low roof van makes it more practical to put stuff on the roof, with some kind of rack. And, the low roof is the only height where it would be practical to gain some access to the roof, by standing on the bumper. So, the bumper is more useful for standing.