I recently acquired this 151d Water Tank Trailer and there are several unusual things about it.
The main thing is the tow ring which is different from other versions that I have. The profile of the ring is circular rather than straight sided and it is also larger in diameter.
Secondly it has a larger diameter axle indicating that it is post-war, even though it still has the smooth wheels. Jacques Dujardin, in his encyclopedia, states that this item was available from 1937 to 1941 and also from 1946 to 1948. However in the "Great Book of Dinky Toys", by Mike & Sue Richardson there is no mention of a post-war version.
Thirdly, it weighs more than the other 2 Water Tank Trailers that I have - 48 grams as opposed to 31 grams.
Lastly it has a flat base with no stamped details.
The other two have "DINKY TOYS made in England by Meccano Ltd" stamped on the base.
I am positive that is not a copy or reproduction, so it would appear to be an early post-war version.
Your water tank trailer may be a white metal copy