Who was exactly present at the door in the ending of the story "The Monkey's Paw"? Was is really the dead and decomposed body of Herbert?

The author of "The Monkey's Paw" deliberately leaves the reader wondering whether the person who has been standing outside knocking more and more insistently is the horribly mutilated and partially decayed Herbert brought back to life by his father's second wish, or whether it is someone else. It could be a coincidence, even in this isolated setting, that an ordinary living man would be knocking. The author intentionally suggests such a possibility in the beginning of Part II.

"Morris said the things happened so naturally," said his father, "that you might if you so wished attribute it to coincidence."

Since Mrs. White couldn't get the door open until after her husband had made his third wish, which must have been that the knocker would go away forever, and since there was no one anywhere in sight when she finally got it open, neither the parents nor the reader could ever know who had been doing that knocking. The reader is left to imagine the horrible creature that could have been standing outside but can never know whether that was Herbert or a figment of the reader's own imagination.

After all, people can't come back to life after they have been mangled by machinery and buried for ten days. Can they? So what other possible explanation is there which would make the knocking a mere coincidence? Well, it could have been some stranger who was lost out there in the darkness and was seeking directions. The author establishes at the beginning of the story that there are only two occupied houses in this new real estate development.

"That's the worst of living so far out," balled Mr. White with sudden and unlooked-for violence; "Of all the beastly, slushy, out of the way places to live in, this is the worst. Path's a bog, and the road's a torrent. I don't know what people are thinking about. I suppose because only two houses in the road are let, they think it doesn't matter."

The stranger might have already tried the other house and found that there was nobody at home. He might be knocking so insistently because he knows there is someone home here. He would have seen the light in the White's bedroom window after Mrs. White had forced her husband to wish for Herbert to return to life.

He sat until he was chilled with the cold, glancing occasionally at the figure of the old woman peering through the window. The candle-end, which had burned below the rim of the china candlestick, was throwing pulsating shadows on the ceiling and walls, until with a flicker larger than the rest, it expired. 

The author inserts that bit of description for the specific purpose of suggesting why a stranger would keep knocking. That light would have been very conspicuous in the otherwise total darkness, and the hypothetical lost stranger would have knocked louder and louder because he was desperate. In other words, it would have been sheer coincidence. The monkey's paw had no magical power but was only a curiosity.

Another possible explanation for the knocking is that Sergeant-Major Morris may have come back for a second visit. He is a seasoned soldier and would not have been deterred by a little bad weather. He has been out of the country for many years and doesn't know anybody else in England. 

Still, we can't help being left with the feeling that it probably really was Herbert, decayed and horribly disfigured, standing out there knocking because he wanted to come back from the grave and live with his parents.