There seem to be three main models of penstyle. There's Janty/Sedansa/ePuffer, there's Njoy/SmartFixx/MyECig/Pillbox/Smokster, and there's e-cig.com/Crown7. The Janty one has four large airholes in the battery end of the atomizer, the Njoy one has two smaller ones, and more white material in the cartridge end, and the e-cig.com one has two small holes, no white material, and an inward curving lip around the inside of the battery connector on the atomizer.
Interchangeability is pretty good between them. The biggest conflict is with cartridges. They all have slightly different barrels and lengths. The e-cig carts will fit both other atomizers, but is a little too short for the Njoy, so won't perform as well as the proper cart. The Njoy cart will not fit in either other atomizer type. The Janty cart will ot fit in the E-Cig.com, and will fit in the Njoy, but poorly, and will not work as well as the proper cart would.
Battery wise, the regular e-Cig.com batteries will fit and work with both of the others, but both of the other types have much better quality batteries, so why bother. The USB, though, might be a good option. It should fit, at least. I don't have one, but Dr. Jim posted about his, and he loved it. He did order another later, though, and it didn't work as well for him. I don't know if that was a bad unit, or a change in design.
