I am trying to figure out exactly what model battery I have so I can purchase cartomizers for it. It is a 21st Century brand ecig. The only marking anywhere on the battery is a "K". I have a 510, in which the threads are slightly smaller in diameter than the 21st Century and a 4081, in which the threads are slightly larger. Does anyone know if there is an ecig that matches up to this? One that falls in between the 510 and 4081?