And yet, RAB must have had an ally of some sort who would have enabled him to acquire the Horcrux in the first place. Dumbledor's solution to the liquid was to drink it. Do you think that Regulus would have been able to either drink it without help, force someone else to drink it, or have enough experience with the dark arts to circumvent a spell put into place by Voldemort himself? Remember, RAB is younger than Severus and probably not quite as well versed in the dark arts, so I think that a little unlikely.

This aid could come from only one of two places - the Order or a traitor within the ranks of the Dark Lord. It clearly wasn't an order member, because if it were Dumbledor would have known the locket was not there. The only other traitor we know of with the knowledge to get past the Dark Lord's defenses is Snape, who even then was a key player. I admit he may not have been the only traitor in the ranks, but he is the only one we know of who could have pulled this type of heist off. But if that is the case, it begs the question, "If it wasn't the real Horcrux, why didn't Snape tell Dumbledor?" Perhaps Snape wasn't as informed of Dumbledor's or Voldemort's plans as we might think.

I suppose Regulus could have hauled someone in there under Imperius, and forced them to drink the liquid for him, but it occurs to me that if you want to leave the Death Eaters you probably would not be using their methods. Especially since you would have to execute the person you had with you in order to keep them from giving you away. (Then again this might be exactly what happened considering Regulus was found out. He just didn't have the stomach for killing anyone else.)

The only other chance Regulus would have had to replace the locket would have been if the Dark Lord had entrusted him with setting up the traps in the cave in the first place. He then could have switched lockets before the traps were instituted. But did Voldemort trust one of his junior members that much? How much would he have told Regulus about the importance of the locket? And why would he have given RAB this info at all? Let's examine the short message in the locket.

"To the Dark Lord, I know I will be dead long before you read this but I want you to know that it was I sho discovered your secret. I have stolen the real Horcrux and intend to destroy it as soon as I can. I face death in the hope that when you meet your match you will be mortal once more. RAB"

From this short passage, we know that Regulus discovered the information on his own. So we are still left with a couple of questions. A- How did Regulus acquire the locket. and B- Is it possible he may have imparted his knowledge to someone else, either on purpose or inadvertantly.

Looking at it from a purely logical standpoint, the only way Snape could have had anything to do with that Horcrux would have been if he and Regulus were taking it to the cave before the defenses were in place. In this case, Snape would have been ignorant of both the objects purpose and the lack of authenticity when Regulus exchanged it. It is kind of hard to believe that one as intelligent as Snape would not have been able to figure this out, but then at the time he was probably still a loyal Death Eater who knew that questions were hazardous to one's health and obedience was king. Still highly unlikely in my mind.

Bottom line, Regulus had to have help from somewhere. It probably wasn't Snape. He and Snape probably only knew each other as black shapes across the crowded inner circle of a madman.