Skewbald:
I should imagine that in specific localities. where munitions and other essential military items were produced, a certain number of tradesmen who would normally choose to join up, were prevented, in order to carry out important war work.
Your G Grandfather would, most likely, be a time-served, Master craftsman and, as such, would be able to make the type of packaging suitable for the transport of munitions. He would also be experienced and skilled enough to train women and also, men who were unsuitable for the military, to produce this packaging.
Although, no doubt, he would have made some of the wooden cases himself, he would probably have been involved in a more supervisory/inspection/training capacity.
Since munitions had to be transported overseas, it was particularly important that they arrived in the best condition. Every case that was damaged in transit would be a local danger because of their explosive contents, and every case lost this way hampered the war effort.
So, your G Grandfather was playing a very important, and probably unsung, role in winning the war.