It is a military 180 gallon water tank trailer chassis from ww2, During the war AC Cars entered into war production and are listed as building fire fighting vehicles and equipment, including " Water Tank trailers" ,as in the pic , various other manufacturers/ contractors built the same thing too in large numbers, not just AC, they were all made to the same standard design ....
.I wonder if the Farmer still has the water tank? ..the only value for such a thing is if it was complete with water tank, pump & filters. Alas as it is just a trailer chassis its only of nominal value, the AC connection really only adds novelty value to it.
AC built other things apart from cars too including several railway trains, including the Southend Pier train