..battle-field expediency...
Only those who have had time in a military unit...especially during actual combat...really understand this fact: whatever gets the job done and works. Water cooled guns used water. Water is scarce in combat. Therefore,you use anything and everything you can get your hands on to carry and store water. Canteens,canvas bags,canvas buckets,steel cans,etc. were used not only in combat but training also. Canvas buckets,water cans and the large canteens were issued to the 1917 Machine Gun Cart crew. About 4 1/2 gals. total. Add another gal or so if the canvas bucket was filled. The canvas bag shown could be Brit,but don't recognize it as a US issue item. Again,anything and everything was used by both sides and all partisipants. Its the same with every war since the first guy threw a stone or swung a stick. Ever hear of the word 'scrounger'..,.? Every unit had a least 1-guy who could find anything and these were the guys who were masters of 'battle-field expediency'.