It was probably constructed in the 1960s, as it originally belonged to my father. Not a canteen-style bottle, but a bona-fide canteen. Imagine two pie pans, one inverted over the other, and welded together at their rims, with the cap hole drilled through the bottom of one pan. That's the basic shape. Smooth out the edges to be more like spherical sections, and put the result in a snap-shut canvas holder, with a shoulder strap.
You have likely never seen the shape I am referring to.
At some point, someone decided to move the caps to the cylindrical sides, rather than keep them on one of the round faces, and then you couldn't fit the thing under a bathroom sink faucet any more, and a drinking fountain needed a bit more arc to fill it. Difficult fill locations might only fill those up halfway. This was likely done to accommodate blow-molded plastic canteens, and to manufacture from flat and rolled sheet metal, rather than pressed.
So now, even if you buy a "real" canteen, the cap is pointing in the wrong direction, and you might as well just have a large, awkward bottle.
You have likely never seen the shape I am referring to.
At some point, someone decided to move the caps to the cylindrical sides, rather than keep them on one of the round faces, and then you couldn't fit the thing under a bathroom sink faucet any more, and a drinking fountain needed a bit more arc to fill it. Difficult fill locations might only fill those up halfway. This was likely done to accommodate blow-molded plastic canteens, and to manufacture from flat and rolled sheet metal, rather than pressed.
So now, even if you buy a "real" canteen, the cap is pointing in the wrong direction, and you might as well just have a large, awkward bottle.