Western Electric 396A

Is there a difference between NOS 50's and 60's WE396A and the 1970's production. I notice that the ones from the 50's and 60's sell much higher than the others. I was wondering what the differences are. Also, they seem to test stronger (from what I read on Ebay).
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That sounds like a reasonable theory. The WE 396A's on Ebay seem to be going to around 100 bucks per matched pair if they are NOS. But then you can get the 70's stock pair for around half of that from companies like Antique Audio supply.
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Thanks, I'll see if I can lay hands on a pair of those to check them against the WE.

I'm using them in a sophia baby tube amp. The sophia guys suggested the WE tubes to increase transperancy and dynamics.