The Fifth Head of Cerberus is famous for having a major twist of identity, but it seems that there is a second twist, perhaps one not identified.

The first twist: the smoking gun

You must excuse my writing in this entry, and I suppose some of the subsequent entries as well. An absurd accident has occurred . . . . I suppose I should have been badly mauled, but I got nothing more than a few scratches from the thorns . . .
The officer laid down the canvas-bound journal and rummaged for the tattered school composition book with the note about the shrike. When he found the book he glanced at the first few pages, nodded to himself, and picked up the journal again. (5HC, p. 233-34)

Following the officer’s lead, we turn back to the composition book passage that we had read over his shoulder earlier, written by V.R.T. as a child: “Birds I have seen today. I saw two birds today. One was a skull-shrike, and the other was a bird that the shrike had [. . .]” (p. 138). Thus, the officer has compared the two handwriting samples and found them to be the same.
As John Clute puts it,

By dint of attempting to read close up to the words, I came to the conclusion that there was simply no reasonable doubt about what had happened to the anthropologist . . . even the exact page — page 233 of the American first edition — where VRT assumes the dead human’s identity seemed to be marked incontrovertibly. (Strokes, p. 159)

The second twist: the third time’s the charm
The first twist is widely celebrated, and it is so clear cut that once you see it, you cannot unsee it. It is helped by this in the fact that the officer sees it and acts as a pointing hound for the hunting reader. The second twist is more obscure, and the officer gives no sign of having seen it.

It amounts to the very different ways a given person reacts to the same stimulus. In “V.R.T.,” the prisoner considers Celestine in three different cases with very different attitudes: misogynistic dismissal; dream re-appraisal; and physical attraction.

Celestine Etienne is introduced in the text when the prisoner writes about the episode of his arrest. In prelude to the action, he notes that the boarding house was unusually quiet that night:

And thus there was no snoring, no one stumbling down the corridor to the lavatory, no muffled sighs of passion from Mlle. Etienne’s room while she entertained herself with the fruits of imagination and a tallow candle. (5HC, p. 168)

This brutal, crude statement shows the misogyny noted by the interrogator in the second interrogation: “It is an obsession of yours that physicians serve merely to keep ugly women alive—you referred to it only a moment ago. And in your notebook you give us a Dr. Hagsmith [an obvious pseudonym by a sneering misogynist]” (p. 206–7). Near the end of the arrest episode, Celestine is brought into the room and the prisoner appraises her physique in disparaging terms: “a very tall girl of twenty-seven or -eight” (p. 173),

[S]he was, as I have said, exceedingly tall, her legs stiltlike in their elongation, rising on thin, straight bones to hips broader than seemed consonant with the remainder of her physique, after which her body contracted again abruptly to a small waist, small breasts, and narrow shoulders. (p. 173)

However, many weeks later, after the prisoner has been broken down in the harsher part of the prison, he considers Celestine in a second way. On this occasion he has awakened from the dream of a woman which sets him to musing on female types which boil down to a single type: “all the things we [men] consider beautiful in a woman are merely criteria for her own survival and thus the survival of the children we shall father in her” (p. 210–11); “And so we seek long-legged girls, because a long-legged girl is swift to fly danger. . . . But a girl too tall will run clumsily” (p. 211). Stirred by the dream woman, his musings can be seen to reflect on measurements that he had used on Celestine the night of his arrest, even though he never mentions her name.

The third case comes on an occasion when the prisoner is taken from his cell in the night and he expects to be executed, but instead he is thrust into a bedroom with Celestine Etienne. She wears “a pink dress without sleeves, white gloves, and a hat. I know I used to think her tall as a stork, but the truth is that she looked a pretty creature there, with her big, frightened, blue-violet eyes” (p. 232). The two engage in carnal relations (p. 233).

This might be taken as a simple case of a broken prisoner finding as attractive a woman he previously sneered at, but there are clues in the text that the anthropologist is a misogynistic homosexual, which simplifies things by assigning misogyny to Marsch. Thus, the prisoner’s initial rude remarks about Celestine are from the Marsch persona; the dream woman and musings are more nebulous; and the third way the prisoner sees Celestine is from the V.R.T. persona.

This sequence is equivalent to the moment when the handwriting changes, but it goes in the other direction: V.R.T. has shed his Marsch persona.