Any thoughts on which (Clarios? or Exide?) plant the Costco H6 AGM originated from?
Like the twin it replaced, it's most certainly a Clarios. Neither is from their Korean affiliates, which didn't become a source until years after 2015, and have distinct case styles. Nor is it a recognizably Exide case.
The three likeliest candidates are China, Mexico, and Germany, probably in that order.
But to them (Clarios), Interstate, and Costco, they're all practical equivalents, with the next shipment, wherever it may have originated, dictating what ends up on the rack.
What stands out to me is the Costco Tire Center barcode, meaning they've implemented an individual unit tracking system for warranty purposes.