The pound of silver is TOTALLY relevant. Thats the reason its called the "pound" note.The pound of silver is irrelevant. Whatever it was worth, that's what the currency bought 300 years ago; that's the purchasing power a person 300 years ago actually had with that currency. It's substantially less than purchasing power today.
You want to take silver and today's market price as if it's relevant to past valuation or some alternative for what modern currency could be worth. It is not. Modern currency could not possibly be made of gold or silver because it would be completely impractical.
By your definition of surviving paper money, If I can find a specimen of Chinese currency that died out 1000 years ago, its still considered a surviving currency, even though it has no tender value.
Do 300 year old pound notes that say the bearer will be paid a pound of silver still legal tender too? If they aren't then they aren't the same pound note.
i think we both know the answer to that question. The current pound note has only been around for less than 50 years. Not 300 like you claim. Just because they call it the same thing doesn't mean it is the same thing.