New Orion model from NASA.GOV

[Mechanic]

Originally Posted by dhanners

Some rocket motors, such as the RL-10 on the Centaur Upper Stage, have a nozzle extension that deploys in flight by sliding down rail thingies. That may be what is depicted in the drawing.

That is so interesting, I've never noticed or been aware of a deployable nozzle extension. I also just watched 'Trial by Fire', NASA's video describing the test flight, and if you look closely you can see those struts/rails on the animation model. I think you're spot on.

Pretty clever solution to save weight as opposed to building the stage adapter two or three feet taller to accommodate a larger bell.