What is this item?

[Marc Montgomery]

Certainly military..

a flat tank, presumably for rad fluid (??)
they are thin, about 1-inch , and roughly 6 in by 10 in
A brass connector on one end to accept about a half inch threaded connnector

while out the bottom (or side?) a hose with a clamp would likely be fitted..

Interesting in that there are no pre-drilled mounting holes, but have flanges on three sides.. would it be slid into place??

any ideas as to application?