If memory serves me right, the 1948 Simplex catalog (in French, I think) shows the earlier dropouts they offered - the touring model with the two-hole fitting in front of the actual dropout, and a racing variant with pretty much a straight line from the rear edge to the bottom of the derailleur ear area. Chuck S. sells reprints of this catalog - in fact, that's where I saw it, and if David G. is reading this, he has my old copy, I think ...
Gist is - the Simplex dropout was around at least that early, anyway. I'm not sure when it took on the form we all know and love from assorted Peugeots and Gitanes of the great bike boom. And then there were the early '80s Simplex dropouts that actually had threaded derailleur ears with the spur like everyone else's to allow use of non-Simplex derailleurs ...
Russ Fitzgerald
Greenwood, SC