Harry Potter and the Half-Blood Prince

Continuity mistake: When Severus talks to Narcissa and Bellatrix in his house, he states that he "had sixteen years of information" about Dumbledore when Voldemort returned. However, Voldemort returned in June 1995. Snape would only have been teaching at Hogwarts (and supposedly spying on Dumbledore) for not quite fourteen years at that point, as he started in 1981.

Question: In the chapter called "Horcruxes", Dumbledore says he thinks Voldemort meant to create 6 Horcruxes. He created 5 of them before attempting to kill baby Harry. Dumbledore then tells Harry, "I am sure that he [Voldemort] was intending to make his final Horcrux with your death." However, as we all know, Voldemort's curse on Harry rebounded, and he lost his powers."After an interval of some years, " Dumbledore then tells Harry, "he used Nagini to kill an old muggle man, and it might then have occurred to him to turn her into his last Horcrux." My question is: if Voldemort was planning to use 1-year-old Harry's death to make his sixth and final Horcrux, but it hadn't yet occurred to him to use the snake as that Horcrux, what was he planning to use instead? He had already used the diary, the ring, the cup, the locket, and the diadem for the first five Horcruxes. So, if he wasn't originally going to use the snake as the sixth, what was he going to use, and what happened to this mysterious object? He must have had something to use, since he went to baby Harry's house fully intending to kill Harry and create the Horcrux. So he must have had the Horcrux object with him. What was it? (Please understand this is not a hypothetical question about what sorts of objects Voldemort might have theoretically wanted to make into Horcruxes someday - I already know the answer to that - but rather a practical question about what Voldemort intended to use that night, the night he went to kill Harry, since he didn't have Gryffindor's sword, or anything else of Gryffindor's that we're aware of).

Aerinah