The most likely answer for this is the Zulu society of southern Africa.
In Chapter 14, Diamond argues that societies amalgamate for two reasons. They can amalgamate under threat from outside enemies or they can amalgamate through conquest. The Zulus were an example of this second way of amalgamation. They amalgamated when Dingiswayo started to conquer other Zulu bands and centralized them under his authority. So, in this way, the Zulu were not really different.
However, right at the end of the chapter, Diamond talks about a way in which the Zulu were somewhat different. This was because Dingiswayo's group had no real advantages over any of the others. They had no different technologies or anything like that. Their only advantage was in their system of government. This was, Diamond says, unusual and is why the Zulus are the society that amalgamated in way that was relatively different.