An original American Army canteen. This is the black enamel variant.
This type of construction was introduced in 1942 due to the shortage of aluminium, when other materials were tested for canteens. The enamelled canteens were made from low grade steel, with the enamel added to aid longevity. However, it was found to be unsatisfactory as if dropped, the enamel flaked away and sometimes, into the drinking water. The low grade steel was then very prone to rusting where the enamel was removed.
This example is stamped U.S. REP 1942 on the underside. The cap is the earlier, flat top bakelite style, consistent with 1942. The cork inside is present.
There are a number of dents and dings as these were very prone to and a couple of spots where the enamel seems to have been retouched.