I'm currently writing my thesis about ZFC origins, and I need a source from where to know who gives the actual first order formulation of ZFC. I got to the point where Bernays writes about it and establishes them, but it seems like a background of NBG. I'm trying also to get access to Skolem's paper of 1929 called Über einige Grundlagenfragen der Mathematik, but I cannot find it anywhere so came here for some help.
Thanks