DSC 0118fi - A surprise Metropolitan Water Board find!

I have never seen any Metropolitan Water Board (MWB) covers except those labelled ‘Fire Hydrant’ or otherwise (these are numerous across London and often stamped with another company’s name, such as Ham Baker Limited, Becks and Company Limited, Blakeborough and Sons or Butterley Limited, etc., these I imagine are related to those companies doing works on behalf of the MWB) or those little flaps which give access to water mains valves.
Yesterday I came across this stupendous manhole cover in a side street near one of London’s famous museums. Its obviously a very old drain cover and the name is unmistakable.
DSC 0118 - A surprise Metropolitan Water Board find!
I’m unsure what the SD stood for.
The MWB lasted from 1903 to 1974 and its headquarters were at New River Head, Islington. The MWB was the forerunner of Thames Water and the buildings in Rosebery Avenue are still there, having been converted to expensive residential properties.