What really matters is the maker of the tube and then if you know that then you can tell what the date code might be.
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Well, Tube World, the high-end boutique online tube dealer, says the Sylvania metal base is regarded by many as the best US made 6SN7. Many will disagree with that, of course. The metal base are more rare so more expensive.
The metal base and many black base Sylvanias are considered excellent tubes. What is printed on the black base?
I have a pristine pair of the "nickel" base Sylvania's which I've used in my Modwright 5400 as well as a couple different pieces of Atma-Sphere gear. Too my ears, they're OK but not worth the money some are demanding for them. I have a couple pairs of the black base version and they sound pretty much the same.
If you've got the budget for the old nickel base Sylvania's, I'd stretch it a bit and look for a pair of the old black glass Tungsols. I have a couple pairs of them and out of my collection of around 60 NOS 6SN7's, they're definitely my favorite, by a wide margin.