looks like some kind of reservoir to me . the flange on the spout would twist and lock in , and the way the throat is stepped it looks like it was made to fit into either a O-ring type receiver or a boot type.
i have been searching for Diesel boat engine fuel traps and oil traps which catch sediment /water and trash then they simply unscrew it and dump the contents out, but i can not find any that match it.
so i am not sure but that is the avenue i am thinking .
just my thougths , not sure what it is , just what it makes me think of,,lol