I aways grab metal sleeves like this when I find them (not very often!) as they're excellent to throw in the car or flea market bag when one is out hoping--always!--to find old drawings and prints. It's definitely old, and oblong rather than round... I assume made for military use holding maps but unusual in form. At 16 inches it's an easy one to carry around, so I think it's just right, and the lid slides on easily and holds tightly closed to well protect whatever's inside.
16 1/8" l x 2 1/2" w x 9/16" d and in very good condition, with plenty of surface scratching to the paint making for what I'd call a great patina.