In trying to trace the history of forcing in an earlier MO questionMO question, I came across G.H. Moore's The origins of forcing. I think you can find in Moore's piece an answer to your question, too. On p. 164 he writes:
From the corresponding paper of Solovay, A Model of Set-Theory in which Every Set of Reals is Lebesgue Measurable, p. 4:
The next page footnotes:
Our original definition of generic was based on "complete sequences". The present approach is due to Levy [8].
I have found no copies of the Levy papers, but Solovay's citations of 8 and 9, respectively, are: