Elaine tells Alex that she almost called her fake boyfriend Carmen Ghia, a character from The Producers (1967). Although it is also possible it is a reference to the Volkswagen model Karmann Ghia, which was the basis for the joke in The Producers.
Elaine's decision to call her fictitious boyfriend "Bill Board" is part of a long tradition in fiction where characters who need to come with a fake name fast improvise one from the first couple of words or things within their immediate line of sight.
In this episode we find out that Elaine's maiden name is O'Conner
There is a long tradition in fiction where characters who need to come with a fake name fast improvise one from the first couple of words or things within their immediate line of sight. After choosing to call her fake boyfriend "Bill Board", she remarks to Alex (who fills in for the fake boyfriend) that he had come "this close" to being Karmann Ghia, the model of Volkswagon she very likely saw on the street while she was driving when she invented the fake boyfriend.