The 24hr assault ration box is synonymous with being used by Airborne troops on D Day. Although very few of these examples exist beyond museum pieces we have been able to faithfully reproduce this item to compliment your impression.

The ration box was designed to go in to the small mess tin, while the 24hr ration box was to fit the large mess tin. The airborne troops would have had one of each prior to a drop or planned operation.
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.