The 24hr assault ration box is synonymous with being used by troops on D Day. 

The 24hr ration box was to fit the large mess tin and troops would have had one issued to them prior to embarkation
The boxes would have been waxed to offer a water and gas proof coating, in my sales i include 50g of paraffin wax to coat yours. As you can see from the photos listed here it makes the box look slightly darker once the wax has impregnated the cardboard.

This is supplied flat packed and ready for you to construct with normal PVA glue. A little tip, dont overdo it on the glue. If you have extra and it dries it can produce a line the wax doesnt hide.  So glue efficently and carefully.