Not sure exactly what it is, but I've been looking for one too that matches the throat hit of the SafeCig..... I think I found one that's pretty close and has a similar taste: Churchill from LibertyFlights. I have so many bottles of
e-liquid that are 3/4 full from trying to replicate the SafeCig throat hit and every stinking method I could find and have been unable to. But in my first few attempts with this Churchill it's been the closest.
For what its worth, I just placed an order for some blank cartomizers from VaporKings to see if maybe this is the answer (SafeCig doesn't really want you refilling their cartos and it is starting to become obvious to me that they really really really don't want you to: hard mouthpiece caps that break when you try and pop them off, very tightly packed cotton - which, as an aside, is maybe why they have such a great throat hit - so that you can't get a syringe in there).