The serial number is at the top edge of the cabinet. That way, if the tag goes away, the serial number is still there. Just a guess, but likely.
Now, to the other part. There's no such thing as a 'balanced' passive speaker. The XLR plug and jack were used when the speaker was used in a portable system and to prevent disconnection. The black paint reinforces this, a bit. Klipsch offered their speakers with or without being finished and this was one of the unfinished ones.
Specs- remember, these were designed in 1957 which, in audio equipment terms, is a long time ago. Having been born that year, I'll take a bit of offense in these being described as 'look to be as old as the earth'.
http://assets.klipsch.com/product-specsheets/Heresy-Spec.pdf