The 51ft container is similar to those used to ferment beer and is believed to have fallen from a ship.
The container was found by a dog walker on Poll Na Crann beach near Griminish - known as Stinky Bay - west Benbecula.
Stornoway Coastguard said it resembled tanks used by the American brewer Coors. The company said it was not aware of a missing container.
No-one at the German firm Ziemann, which makes the tanks, was available for comment.
A number of people contacted the BBC news website with theories about the origins of the container.
One reader thought the numbers "5580" written on the container could have meant it was used to hold chemicals in DTD 5580, a three-part paint aircraft scheme.
Another writer thought it could have been a rocket stage launched from South Uist, from which meteorological rockets were fired in the 1960s and 70s.
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