Actually, it's more complicated than that. There is no way to way to know if the last three pages or so of the story were rewritten by the editors or by Howard himself. I'd tend to think REH himself did the rewrite, exactly in the same fashion he did for several contemporary stories. I'll email you the details later today.
Interesting, because I think the published version is much weaker by hitting the reader over the head definitively with the id of the apparition. I think the original version works better as it's more subtle and keeps a sense of ambiguity (even though it's still clear who the apparition is). It just seems like the kind of change an editor would suggest to sort of "dumb it down." Look forward to that email.
Also, I see mentioned that it's in the Best Of collection, but I meant to say the Horror Stories collection.