Thank you very much for all your comments and queries. I realise from reading the comments that I haven’t been able to articulate the fundamental reason for my even beginning to look at this area.
And that is this:
The original Dunbar number was based on some understanding of the relationship between neocortical volume and group size for nonhuman primates.
Humans seemed to scale differently, and research suggested that from a group size viewpoint, the root cause for this difference was language.
The move from oral to written communication is also meant to have aided this process, as man learnt to store and retrieve observations and learnings about the group.
My contention is that anything that aids and improves group communications may also have the effect of raising the “theoretical” maximum for the size of group.
Maybe what I’m saying is that the Dunbar number is a constant, yes, but only in the context of a specific class of primates, a specific set of languages and linguistic abilities, and a specific set of communications processes and technologies.