The chocolate and other items in these 'emergency' or 'lifeboat' rations contained no (or very little salt) so that you wouldn't fancy a drink afterwards unlike everyday chocolate. They seem to last for ages so I wouldn't mind betting that yours would still be perfectly edible. My old friend who died recently told me when I was looking into them that the last time he ever saw them issued was as 'operations' kit during ops in Korea where they could be used for escapers and evaders.
As a matter of interest, our now obsolete ration packs also contained chocolate bars that tasted 'different' to what was available in the shops (Yorkie bars.......) These were salt free too
The same VERY limited saline content also applies currently to the drinks, especially the powdered variety if anyone's ever wondered why they taste nothing like anything in the commercial world