I'm not sure when it was first mass produced, but Vaporesence often claims to be the first original manufacturer of the cartotank, with the traditional end caps and o-rings.
Not long after, Smoktech made their entry into the market, and included one of their new dual coil cartomizers as part of a marketing program. The tank was called a DCT, or dual coil tank. The marketing program and combo obviously was a success, as a generic XL length cartotank is now synonomous with DCT, much like Bandaid is with adhesive bandage and Q-Tip is with cotton-tipped applicator sticks.