Casting a look at the Mushaf, which is a composite of the ‘suhuf mutahharah, i.e. purified pages’ conveyed by the Messenger, peace be upon him, we find that it is composed of 114 suras, starting with Sura al-Fatihah (No. 1) and ending with Sura al-Nas (No. 114). This is what I here call ‘the Glorious Mushaf’ or ‘the purified pages.’
These suras of the Mushaf are in two divisions:
1. Suras that do not start with isolated letters, and they are 85 in number; and
2. Suras that start with isolated letters, and these are 29 in number. The following chart illustrates their structure, each with a number.

See Figure (2a-2b)

So, these isolated letters are encountered at the beginning of some suras. Now each messenger or prophet who received revelation had his message started with one of these isolated letter/letters, immediately followed by something else. We have, in support of this, the following verse from the Mushaf:
“Ha-mim-‘ayn-sin-qaf. Thus, Allah sends inspirations to you as He did to those before you – Allah, Exalted in Power, full of Wisdom,” (42: 1-3).
It is the isolated letters that identify the Books and the prophets or messengers to whom they were revealed. They are the sign of the prophethood: every prophet or messenger was given one of these letter/letters, in addition to something else, maybe more than one thing.
The Arabic language has 28 letters, half of which have been used as isolated letters. The letters used as isolated letters are:

Alif – lam – mim – ra’ – sad – kaf – ha’ – ya’ – ‘ayn – ta’ – sin – ha’ ‘letter next jim in the Arabic alphabet’ – qaf – nun

See figure 3 where the letters used as isolated letters are listed, half of the entire number of the Arabic alphabet.

See Figure 3

 

There are fourteen variations in the Mushaf of the fourteen letters, viz:
Alif-lam-mim, alif-lam-mim-sad, alif-lam-ra’, alif-lam-mim-ra, kaf-ha-ya-‘ayn-sad, ta-ha, ta-sin-mim, ta-sin, ya-sin, sad, ha-mim, ha-mim-‘ayn-sin-qaf, qaf, and nun.
Of these suras that start with isolated letters, six start with alif-lam-mim, namely: suras No. 2, 3, 29, 30, 31, 32; six start with ha-mim, namely: suras No. 40, 41, 43, 44, 45, 46; five start with alif-lam-ra, namely: suras No. 10, 11, 12, 14, 15; two start with ta-sin-mim, namely: suras No. 26, 28; only one with alif-lam-mim-sad, namely: Sura No. 7; only one with alif-lam-mim-ra, namely: Sura No. 13; only one with kaf-ha-ya-‘ayn-saad, namely: Sura No. 19; only one with ta-ha, namely: Sura No. 20; only one with ta-sin, namely: Sura No. 27; only one with ya-sin, namely: Sura No. 36; only one with saad, namely: Sura No. 38; only one with ha-mim ‘ayn-sin-qaf, namely: Sura No. 42; only one with qaf, namely: Sura No. 50; only one with nun, namely: Sura No. 68. The total is 29 suras starting with isolated letters.

(See figure 4).

 

The above 29 suras are actually 29 Books: each refers to a specific Book, and the referent of the Book is found in the Glorious Mushaf itself.

See Figure (5)

It will be noticed that the Books of each prophet start with one of the isolated letter/letters, in addition to something else for designation.
The above 29 suras with isolated letter/letters were revealed each to a specific messenger or prophet, in addition to another designation. For evidence, we have the following quote from the words of the Almighty:
“Ha-mim, ‘ayn-sin-qaf. Thus He sends inspiration to you as He did to those before you – Allah, Exalted in Power, full of Wisdom,” (42:1-3).
Here the Lord is saying: ha-mim, adding: ayn-sin-qaf, and then He adds, addressing the Prophet: Kadhalika, ‘Thus’: that He is revealing to Muhammad, peace be upon him, in the same way as He had revealed to the previous prophets.
Here is an example to show that the separate letter/letters designate a specific Book:
“Alif-lam-mim-saad. A book revealed unto you – so let your heart be oppressed no more by any difficulty on that account – that with it you might warn the erring and teach the believers,” (7:1-2).
From this we realize that alif-lam-mim-saad is an unidentified Book; but its identification comes in another location: it does not remain a mystery; it is one of God’s Books.
This Book, referred to with alif-lam-mim-saad, is definitively identified in the same Sura 7, in the verse:
“This is insight from your Lord, and a guidance and a mercy for a people who believe,” (7:203).
This insight is al-Zabur, the Psalms, and we get this by inference from the following verse in Sura 105:
“Before this We wrote in the Zabur ‘Psalms’, after al-Dhikr ‘Message, given to Moses’: ‘My servants, the righteous, shall inherit the earth,’” (21:105)
By looking at both the above verse ‘Before this We wrote …’ and Verse 100 of Sura 5:
“To those who inherit the earth in succession to its previous possessors, is it not a guiding lesson that, if We so willed, We could punish them too for their sins, and seal up their hearts so that they could not hear?”
We find: alif-lam-mim-sad is a Book; this Book provides insight; this insight is the essence of the Psalms: we get this by inference from Sura 21, which provided a sign to designate this Book, the Psalms, and this sign is met with in Sura 7. Then, from Sura 4, Verse 163: “and to David We gave al-Zabur ‘the Psalms’”, and Sura 17, Verse 55: “We bestowed on some prophets more gifts than on others, and gave to David the Psalms,” we infer who the Psalms was revealed to: it is to David, who is addressed in the Sura in hand, No. 7.
From all this it becomes manifest that Sura 7 is the Psalms, which was revealed to David; and it is David that is addressed in this Sura. Muhammad is mentioned in it as a third person.
Here is another example:
We recite in Verse 1 of Sura 14:
“Alif-lam-ra. A Book which We have revealed to you, in order that you might lead mankind out of the depths of darkness into light – by the leave of their Lord – to the Way of Him, the Exalted in Power, worthy of all Praise!”
It is seen here that alif-lam-ra is a Book; this Book is to lead people out of darkness into light. Now we know that it is Moses who was sent to lead people out of darkness into light: we know this by the evidence of Verse 5 of the same Sura: No. 14:
“We sent Moses with Our Signs and the command: ‘Bring out your people from the depths of darkness into light, and teach them to remember the Days of Allah.’ Verily, in this there are Signs for such as are firmly patient and constant – grateful and appreciative.”
We infer from this that it is Moses who is addressed to bring his people out of darkness into light. This is what we find in Sura 14.
Let us now read Verse 105 of Sura 21:
“Before this We wrote in al-Zabur ‘the Psalms’, after al-Dhikr ‘the Message, given to Moses’: ‘My servants, the righteous, shall inherit the earth.’”
And by comparing it with Verses 13-14 of Sura 14:
“But their Lord inspired to them: ‘Verily, We shall cause the wrong-doers to perish; and verily We shall cause you to abide in the earth and succeed them. This for such as fear the Time when they shall stand before My tribunal – and such as fear the punishment denounced.’”
We learn from this verse that al-Dhikr was bestowed on Moses; and we learn that al-Dhikr is a Book, as supported by Verse 41 of Sura 41:
“Those who reject al-Dhikr ‘the Message’ when it came to them are not hidden from Us. and indeed it is a Book of exalted power.” We infer from the above that al-Dhikr was revealed prior to al-Zabur, or the Psalms: now we all know that Moses was before David.
Speaking more generally, these isolated letters are signs or indications of specific Books; definitely not, as the commentators claim: charms, or Assyrian, Aramaic, or Ancient Egyptian characters.

Locations with prostrations:
There are in the Glorious Mushaf 14 suras with prostration at particular verses; seven of which start with isolated letters, namely:
Sura 7, starting with alif-lam-mim-saad;
Sura 32, starting with alif-lam-mim;
Sura 13, starting with alif-lam-mim-ra;
Sura 41, starting with ha-mim;
Sura 19, starting with kaf-ha-ya-‘ayn-saad;
Sura 27, starting with ta-sin; and
Sura 38, starting with saad.
Seven of the suras with a prostration sign do not start with isolated letters, except for al-Hajj, which has the prostration sign twice, namely:
Sura 84, Sura 96, Sura 53, Sura 16, Sura 17, Sura 22 (with two prostrations), Sura 25.

See Figure 6