It's the Hobby King 8,000 mAH 5S Lithium Polymer battery.
With a pair of these packs, for an equivalent 10S1P, I can get 37V 8AH which considering the cost ($74 each, $148 a pair) is quite an amazing bargain. They are also capable of very high current output and can be charged with the pair of balance plugs. Gary Goodrum's BMS should be perfect for this setup when configured for the 5S packs and it can simply be placed into the hub with only the power plug extending from the hub itself. Very neat and very powerful considering the cost and size...