I seem to remember (someone correct me if I am wrong) that they first began to use a term 'approved associate', which lasted maybe a year or two, then that designation was replaced by 'unbaptized publisher'.
I think it came about because of the inherent ambiguity of the WT definition of 'who is a member'. First they say a member is someone who is baptized, then they would say a JW is someone who reports time, ie; a publisher. The two designations do not always jive, so they had to come up with an 'unbaptized publisher' designation. Part of their strategy to keep one step ahead of the draft board in the case of young men facing selective service, in my opinion.