Edited to add screenshots of parts of the letter in question at the end of this answer.
The most likely answer is (e), something else entirely. The heading to this letter says that it was transcribed from manuscript (MS) rather than typescript (TS). The transcriber has probably misread the word.
In Excellent Women, the narrator, Mildred, recounts her first encounter with her new neighbor, Helena:
"You'd hate sharing a kitchen with me. I'm such a slut," she said, almost proudly.
and a little further on:
I hope you don't mind tea in mugs," she said, coming in with a try [sic]. "I told you I was a slut."
Pym, Barbara. Excellent Women Ch. 1. 1958. Introduction by A. N. Wilson. New York: Penguin, 2006. p. 4.
Helena means slut in the first sense given in the OED:
I.1. depreciative. An untidy, dirty, or slovenly woman; a woman who is habitually careless, lazy, or negligent with regard to appearance, household cleanliness, etc.; a slattern. Also rarely applied to a man. Now somewhat dated.
“Slut, N., Sense I.1.” Oxford English Dictionary, Oxford UP, September 2023, https://doi.org/10.1093/OED/3665174539. Accessed 28 January 2024.
She does not mean it in the sense more common today:
I.2.a. Chiefly derogatory and offensive. A sexually promiscuous or lascivious woman; (also) a female prostitute. In earlier use also: a vulgar, impudent, or disreputable woman. In recent use also occasionally applied to a man.
“Slut, N., Sense I.2.a.” Oxford English Dictionary, Oxford UP, September 2023, https://doi.org/10.1093/OED/1116694172. Accessed 28 January 2024.
The rest of the novel provides plentiful confirmation of Helena's being a slut in the now outdated sense of slatternly. On one occasion, Mildred smells something burning in Helena's kitchen and goes in to check:
I found one of the gas rings full on and on it a saucepan of potatoes which had boiled dry and were now sticking to the bottom in a brownish mass. I dealt with them quickly but the saucepan was in a very bad state. I ran some water into it so that it could soak. I noticed with distaste and disapproval that the breakfast things and what appeared to be dishes and glasses from an even earlier date were not washed up. The table by the window was also crowded; there were two bottles of milk, each half-full, an empty gin bottle, a dish of butter melting in the sun, and a plate full of cigarette stubs.
Pym, ibid., Ch. 17. p. 136.
It's easy to imagine how the word slut, in cursive handwriting, could be misread for shit. I believe that whoever typed out the letter from Larkin's manuscript for Thwaite's collection made this mistake, and that Thwaite failed to notice it.
I cannot find an image of the manuscript of this letter online, but available samples of Larkin's handwriting do make this explanation plausible. For example, here is a page from a letter to his longtime girlfriend Monica Jones:
![Image of letter from Larkin to Monica Jones](https://cdn.statically.io/img/i.sstatic.net/17Js8.jpg)
Retrieved from Layabout, kekasden.blogspot.com, 28 January 2024
The repeated word sure and the word should in this reproduction of Larkin's holograph shows that his cursive could theoretically be misread as *sme / *smi or *sliorild respectively. Those aren't real words, so the context is sufficient to ensure that the word is read as intended. Further, the word shit does occur frequently in Larkin's letters to his other correspondents, appearing 42 times in the archive.org search results within Thwaite's collection. Slut, by contrast, appears nowhere therein. So for slut to be misread as shit would not be surprising. It is a safe hypothesis that this is what happened here.
Edit 20240305. Following @shoover's suggestion in the comments, I acquired a facsimile of the letter in question from the Pym archives at the Bodleian. The word is indeed mistranscribed. Here is a screenshot of the passage in question:
![Screenshot of portion of the manuscript of a letter from Philip Larkin to Barbara Pym](https://cdn.statically.io/img/i.sstatic.net/qCxzL.png)
are. I never see any Rockys, but almost every young academic wife ("I'm a slut") has something of Helena.
Larkin, Philip. Portion of letter to Barbara Pym, 18 July 1971. From the Barbara Pym archives, Bodleian Libary, Oxford. MS. Pym 151, fol. 74r.
It is easy to see how slut could be misread as shit. The similarity is notable when in the same letter, Larkin writes:
![Screenshot of different portion of letter from Larkin to Pym.](https://cdn.statically.io/img/i.sstatic.net/HJ59U.png)
passing my driving test — things such as these? Life doesn't work that way.
ibid., fol. 73v
The hi of things looks very like the lu of slut. But we can confirm it's intended as slut and not shit by looking at the following fragment from the same letter:
![Screenshot of yet another portion of the Larkin-Pym letter](https://cdn.statically.io/img/i.sstatic.net/Y67wW.png)
blessings of my advancing age is a merciful blurring of the sounds
ibid., fol. 73r
The similarity of the lu of blurring to the corresponding letters in the prior examples provides the needed confirmation that lu and hi are easily misidentified in Larkin's handwriting. The hypothesis can be regarded as confirmed: Larkin is indeed paraphrasing Helena's words, and the letter reads I'm a slut rather than I'm a shit.