Thanks for your reply Trevor. I think you may have provided the solution: an editorial slip-up.
While I'm reluctant to suggest that the esteemed Mr. Parker has erred, it certainly makes more sense to me that the clue for 55D was intended as an option for 54D, and that the actual clue for 55D is missing.
Thanks for pointing that out! It might even have helped me to feel a little less stupid, if not for the fact that I somehow double-posted my original question! (For which I apologize to all!)