The package prefix 'N' has never been used in any of the pre WWII and WWII ammunition package marking systems.
By 1968, as marked on the package, there were RN packages with a letter 'N' prefix, for dedicated naval stores.
I do not know whether such packages were for munitions only, or whether this particular package is a general use naval package, which could reasonably be made to hold sandbags, among other things.
What has to be considered is that it is RN policy, that if it could go aboard ship, it must be packaged as though it will be.
What in Land Service use, i.e. sandbags, would be Water, Rot and Fire retardant treated, then tied in bundles to go into storage, for however long, would in naval service, have to be protected from salt water/spray, just in case they went aboard ship.
