How can I tell by serial number?
Got me. I've seen early T's with higher serial numbers than later T's... and vice versa. Nothing consistent as far as I have been able to determine... which hasn't exactly been an exhaustive search by any means.
The most accurate method is the test sticker you may find on the side of the chassis ("may" assumes it hasn't been removed). Without a sticker, the fallback of source/date codes on pots and transformers can get you into the ballpark.
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