Why would he have to have one?

The letter says that he will be dead by the time Voldemort finds out. Regalus doesn't have to necessarily be alive now to be RAB, he just has to have taken Voldemort's horcrux from the cave. It's probably going to be how Harry finds out where the horcrux is, by finding out that RAB is Regalus, then looking wherever Regalus would keep it.

Whoever RAB is, he had to be a follower of Voldemort, because the letter is addressed to "Dark Lord" and nobody called him that but his followers.