The photo is showing the - end of the battery. The shiny spot at roughly 2 o'clock on the outer rim is where the battery spring contact pierced the plastic battery wrap. While I never inspected the batteries very closely apparently the metal casing of the battery is positive, with the negative electrode floating centered in the big end.
The spring with a sharp edge from the coiled wire apparently pierced the insulation and shorted the battery. The initial symptom was some extra distortion at the very LF end of the frequency scan, so apparently low battery voltage. Which I confirmed with a quick measurement across the 4 cell battery pack.
Then I noticed unusual heat coming from behind the top PCB. In over a decade of building these and probably hundreds of these exact batteries, this is a first.
Since one speaker sounded nastier than the other, I swapped the speaker and battery. FWIW the tuner, correctly completed the original scan and read the correct note (I have a lot of headroom engineered into the PS), but I suspect the shorted battery would have caused more mischief over time.

It looks like I have a new concern when assembling these, make sure the spring contact is centered. Only we can prevent house fires.

JR