What were those containers attached to the helmets of soldiers in Vietnam? They looked like they were made of plastic. I’ve never seen them in WW II films or movies.
Water bottles.
The green ones that were square were standard issue bottles of insect repellant.
Sometimes the square ones were soap.
Concur with Turbo Dog.
Turbo and Heretic are both right – they started out as insect repellent and became containers of virtually anything, from C4 to heroin to matches to favorite Biblical passages.
Probably not water, given the bottles’ small size and heritage, but there may have been some guys who were daring enough to try.
Another common thing stored in them, for the hygenic soldier, was mouthwash, which was often accompanied by a tooth brush.