NATO Stock Number
A string of digits that identifies an item uniquely in the stores system.
The book that lists such things is guarded carefully by Blanket Stackers, because knowledge is power. Allegedly there's an NSN for Challenger 2, but I suspect the Brigade Logistics Team would have a word or probably two (very short ones, second word 'off') to say about any attempt to order one up because you fancied a cabbie.
See here if you're sufficiently bored.