Bottle, water, aluminium.
In the final years before the outbreak of the Second World War, the British army was looking at a variety of designs for updating its uniforms, weapons and equipment. One area they looked into was replacing tinned steel mess tins and enamelled water bottles with aluminium alternatives. Aluminium was lighter, shaving a few ounces from the soldiers burden; and more hygienic it didn’t rust:
The water bottle is based on the earlier enamelled version, with a flat top:
The cork is the standard type used on all British water bottles of the period, attached to a string that attaches through a loop next to the spout:
On the base is a maker’s mark and date, M.M.S. 1939:
The bottle is painted in an overall drab green paint and no felt cover was issued with these bottles.
These bottles were never very common, with the outbreak of the Second World War curtailing their introduction. They were withdrawn and recalled for salvage in 1941, with aluminium being considered a strategic resource. Therefore surviving examples are becoming increasingly hard to find. The green army version is more common, whilst I have only ever seen a blue painted RAF version once on a dealer’s website. Needless to say it was not cheap!
Tales from the Supply Depot 
I have a similar bottle and I have always been curious about the maker, MMS. I just came across this link and I now believe that these bottles were made by Midland Metal Spinning Company, Limited. At the 1937 British Industries Fair, Midland listed domestic aluminium hollow-ware in their products.
http://www.gracesguide.co.uk/Midland_Metal_Spinning_Co
That sound’s a very plausible manufacturer. I find Grace’s Guide very useful as well in finding out a bit more information on these long gone companies. The internet and the proliferation of sites covering different aspects of history has been a godsend to the researcher, this sort of information just wouldn’t be worth the effort of hunting in a traditional archive but is very gratifying to have nonetheless.