As an aside: the note to 'Old Spencer' recalls Jerry Salinger's letter to Whit Burnett in late 1939; the 1950 revision likewise suggests Salinger's festering grudge as the reason why Old Spencer has been turned into a nasty villain.
Holden Caulfield is, of course, one of the most famous characters of American literature. Is he also America's favorite fictional fuck-up? The Catcher in the Rye  has sold approximately 75mln copies. So what's the real problem here? Basically, there's no redemption - it's just a classic trainwreck and (exquisitely?) Post-War Jewish-American wank: half-Kafka, half-Hemingway.
Another elephant-in-the-room is the Author's occult and disturbed Jewish identity. This is four years prior to Salinger's single, brief, recorded PTSD episode. The (c.1951) Narrator, in a sanitarium pretending he has 'tuberculosis' [code word: Alcoholism], has had a partial psychic collapse: the story is an attempt to rationalize WHY. I think the topical examination of two versions of a single passage for altered details may begin to reveal the unspoken and still misunderstood underlying cause WHY.
Like several other unpublished 'Caulfield' stories, the 1946 short story, "I'm crazy" was probably written in earlier draft c.1940/1 (??); it was adapted for The Catcher in the Rye c.1949. I will jump straight to my conclusion. In the earlier essay, 'Egyptians' looks like a coded reference to Salinger's Jewish identity crisis (i.e. flight from Judaism/'Sojourning in Egypt' = Insanity). But the revised passage in his published book has removed (all?) those sly Semiticisms and the alchemical details (i.e.Orientalism), erasing the key point altogether. Ancient (Jewish) allusion has been replaced with a Modern (WASP) focus, further obscuring the Narrator-Author's own less identifiable 'Jewish crisis'. In the revision, readers have lost all information about what this is really about: an existential conflict between secular modernity (i.e. 'America') and rituals & tradition of the ancient people (i.e. 'Egyptians'), where Jerry/Holden ultimately mocks and disavows the race and history (i.e. his Jewish background).
It's cryptic, yet equally obvious: who are "the ancient race" who "interest us" and are "read about in the Bible" telling "amusing anecdotes" about "Pharoah"? Here, their mysterious "religion" is "being buried dead... [entombed]", leaving "doctors" to ponder relic artifacts of their alchemy (i.e. preservation for the afterlife).
No: it's really NOT about the "Egyptians" - the "people" who Salinger has "chosen to write about" (i.e. the Chosen People) lie just beyond the pale. Summoning ghosts, his morbid treatment is also indicative of his attitude. One imagines that an atheistic young Jerry actually failed out of the McBurney School in 1934 by writing a moronic (i.e. goyish, hateful) essay on the "Caucasian" Jews and for which his (WASP) teacher rightly scorned him. Yes, we get it: the eternal Jewish Mind is an interesting riddle for modern science of the 20th C., and Salinger is autobiographically working through his precious neuroticism. He never wrote another novel: this was his ONE story, his own painful, shameful failure. And that, dear readers, was very consciously and successfully spun into gold.
** For me today, the theme of Salinger's morbid, buried identity is of interest by counterpoint to another book which also sold in the tens of millions, written 3 years before his (1941) short story. By contrast - arguably, the exact inverse of Salinger's 'Un-becoming' - my comparison text cryptically promotes Judaic conversion (i.e. re-constructed Judeo-Egyptian Metathesis = Sobriety) through a deep analysis of and reconciliation w/ Mosaic myth. Since Exodus is quintessentially Egyptian AND a Jewish metaphysics of 'Becoming', this Jewish-Egyptian conflation is profound. So too in Salinger's first version, we can dimly perceive 'Moses The Egyptian', "Old Pharaohs" = the Jewish Patriarchs, and so on; his contemptuous riddle/"trick" is on us "morons". I am not a fan of that negative ploy, however. Rather, the goal of a responsible, hopeful, pro-active and edifying 'reconstruction of the Temple' is preferable to its wanton, nihilistic and adolescent destruction: in other words, focus on the recovery Solution over the degenerate Problem.**
"I'm Crazy", Collier's 12/22/1945:
From Catcher in the Rye (1951):Old Spencer handled my exam paper as though it were something catching that he had to handle for the good of science or something, like Pasteur or one of those guys.
He said, “We studied the Egyptians from November 3d to December 4th. You chose to write about them for the essay question, from a selection of twenty-five topics. This is what you had to say:
“ `The Egyptians were an ancient race of people living in one of the northernmost sections of North Africa, which is one of the largest continents in the Eastern Hemisphere as we all know. The Egyptians are also interesting to us today for numerous reasons. Also, you read about them frequently in the Bible. The Bible is full of amusing anecdotes about old Pharaohs. They were all Egyptians, as we all know ’.”
Old Spencer looked up at me. “New paragraph,” he said. “ `What is most interesting about the Egyptians was their habits. The Egyptians had many interesting ways of doing things. Their religion was also very interesting. They buried their dead in tombs in a very interesting way. The dead Pharaohs had their faces wrapped up in specially treated cloths to prevent their features from rotting. Even to this day physicians don’t know what that chemical formula was, thus all our faces rot when we are dead for a certain length of time.’” Old Spencer looked over the paper at me again. I stopped looking at him. If he was going to look up at me every time he hit the end of a paragraph, I wasn’t going to look at him.
“ `There are many things about the Egyptians that help us in our everyday life,’ “ old Spencer said. Then he said: “The End.” He put down my paper and chucked it towards the bed. He missed. The bed was only about two feet from his chair. I got up and put my exam paper on top of the Atlantic Monthly.
“Do you blame me for flunking you, boy?” old Spencer asked me. “What would you have done in my place?”
“The same thing,” I said. “Down with the morons.”
Amenhotep I 'Digitally Unwrapped' as reported 12/28/21:"We studied the Egyptians from November 4th to December 2nd," he said. "You chose to write about them for the optional essay question. Would you care to hear what you had to say?"
"No, sir, not very much," I said.
He read it anyway, though. You can't stop a teacher when they want to do something. They just do it.
The Egyptians were an ancient race of Caucasians residing in one of the northern sections of Africa. The latter as we all know is the largest continent in the Eastern Hemisphere.
I had to sit there and listen to that crap. It certainly was a dirty trick.
The Egyptians are extremely interesting to us today for various reasons. Modern sciences would still like to know what the secret ingredients were that the Egyptians used when they wrapped up dead people so that their faces would not rot for innumerable centuries. This interesting riddle is still quite a challenge to modern science in the twentieth century.
He stopped reading and put my paper down. I was beginning to sort of hate him. "Your essay, shall we say, ends there," he said in this very sarcastic voice. You wouldn't think such an old guy would be so sarcastic and all. "However, you dropped me a little note, at the bottom of the page," he said.
"I know I did," I said. I said it very fast because I wanted to stop him before he started reading that out loud. But you couldn't stop him. He was hot as a firecracker.
DEAR MR. SPENCER [he read out loud]. That is all I know about the Egyptians. I can't seem to get very interested in them although your lectures are very interesting. It is alright with me if you flunk me though as I'm flunking everything else except English anyway. Respectfully yours, HOLDEN CAULFIELD.
He put my goddam paper down then and looked at me like he'd just beaten the hell out of me in ping-pong or something.