The paper [History of Lambda-calculus and combinatory logic](http://www.users.waitrose.com/~hindley/SomePapers_PDFs/2006CarHin,HistlamRp.pdf) by F. Cardone and J.R. Hindley is a good starting point for answering such a question, and many others (it has 38 pages of bibliography). There’s a brief account on fixed-point combinators on page 8, although it doesn’t seem to settle the question completely.