I think the whole thing may be the [main members of the] Metropolitan
Board of Works. There's something the same or very similar on at least
some of the old pumping stations.

That would make a lot of sense. The City of London, City of Westminster,
and counties of Middlesex and Kent would certainly seem to be represented.
There would seem to be a lack of representation of Surrey which might
perhaps be a clue to where the remaining two arms might be from.

G.