Cousin Lefty:
I may be dense, but I'm not sure if I understand your question. If by "lip" you mean the lighter colored wood, that appears to be a wale and is meant to stand proud of the regular planking. A wale is a thicker plank that is usually placed in an area of the vessel subject to greater impact or wear.
Fair winds!
Did a little research and found that HMS Terror has a unique history. Initially she was a bomb ship and then quite later refitted as a polar exploration vessel. The structure in question was part of the modification: a very large ice channel to protect against arctic ice. A log on modelshipworld.com by "erebus and terror" presents a goldmine of information on research and modeling of Terror. The pic below illustrates that modeler's approach to constructing the ice channel, that is, including it as part of each frame. Note the unusual cross section shape of the channel.