Very Large Vintage Boyco Canteen

This canteen is much larger than a the usual canteen. A US Army canteen is in photo 3 to show scale.
From what I saw on the net these are rather old. Patent dates on other models are from 1917 or 1919. This could be earlier or later.
Originally I thought this may be a canteen carried by a horse or mule. It would be a bit heavy for a human. Anyone carrying this much water would probably carry a few smaller canteens. This one would put too much pressure in one spot.
My new theory by me is that it would be for a car. Radiator leaks were common in early cars and they often carried water bags. Could aslo be for the passengers who may also have needed water.