Good size hive. How often do you see bees build comb bottom up, like those over that top plate?
Most of what I come across is conventional or semi conventional construction so wall voids are usually framed correctly for the most part.
Bees tend to build from the top plate down in most cases but I've seen them occasionally build from the bottom up, usually small colonies though, hardly ever big booming colonies.
Now plaster walls can be quite different as bees will often just pick a spot somewhere to begin building. Their high point could be anywhere in a plaster wall due to old style/unorthodox construction and different lathe types.
This one with this make shift wall, the high point was actually that 2 x 4 its just that it had space behind it for them to build surplus honey.
Everything that was above the 2 x 4 was surplus honey.