Steve,
You are quite right. I am now thinking yours may be an actual British issue extinguisher. There are slight differences in the lettering, mainly in the prominence given to the Queens St.London address, which was really only a distribution centre during WW1. I was not aware of this detail and this is the first of this type I have seen.
The label, normally brass, appears to be aluminium on your example which was presumably a wartime expediency, there being a massive demand for brass in the munitions industry as America and Canada were supplying a good chunk of the ammunition we were needing on the Western Front. The casing was still brass though. From your pictures it would seem there are traces of a black background to the larger labels, with the indented lettering picked out in red.This is the same as my earlier one.
Both my examples have come from the US and although period correct, were probably intended for the home market.
Tomo