This appears to be a an Her Majesty’s Prison/Borstal water carrier, also known as a chaggle, made from a tight weave hessian. It is in new and unused condition, even through it is dated 1963. The two webbing straps and loop appear to be to allow it to be carried as part of a military style set of webbing. It has a draw string neck. Overall length including straps 31 inches, the “bag” is 9 inches wide by 19 inches long from the top of the neck to the base.
I would be grateful for any information regarding this item.