Before expanding on this well elaborated theory, it should be noted that in spite of the research and proposals by numerous contemporary Western scholars which presume the Muqaṭṭa’āt to be redactional ciphers, not present in the original revelation, an oft-quoted ḥadīth collected by at-Tirmidhī would seem to contradict this assertion:
“Abdallāh bin Masa’ūd narrated that the Prophet said: ‘He who reads one letter of the Quran it (becomes) for him a good deed, and a single good deed is rewarded (by Allah) by ten times the like thereof. I am not saying that Alif-Lām-Mīm is one word, but that Alif is a (separate) letter, Lām is a (separate) letter, and Mīm is a (separate) letter.’”
Noldeke was the first to propose the Muqaṭṭa’āt as abbreviations of redactors who Zayd bin Thabit had consulted regarding the variant readings of the revelation. He later changed his mind, concluding that the letters were mystical symbols observed by the Prophet in the late Meccan and early Medinan periods.
Hartwig Hirschfeld expanded on Noldeke’s theory and proposed a list of amanuenses for the letters:
S=S‘ad bin A’bī Waqqaṣ
Q=al-Qasim bin Rabī‘a
Alas, such proposals are testament to the resourcefulness of their authors rather than providing concrete solutions to the presence of the Muqaṭṭa’āt. Although such abbreviations for the redactors seem plausible, he does not offer an explanation as to why only 29 chapters have been inscribed and not the other 85.
Massey made a comparison of 11 different multiple instances of the Muqaṭṭa’āt. His research revealed that the order of the letters is neither random nor arbitrary, as one would expect had the letters stood for sentences or words.
Sūrah 42, with HM/‘SQ, appears to violate the principle of the letter ranking (the Mīm appears before the Sīn, which does not occur in any of the other co-occurrences of the Muqaṭṭa’āt). Yet, these letters are generally divided into two sets, with ḤM appearing in verse 1, and ‘SQ appearing in verse 2. Massey suggests that ḤM was added by analogy later as this group of Muqaṭṭa’āt appears in the middle of a group of chapters prefixed by ḤM.
The discovery of the ranking system for the letters adds further weight to the theory that the Muqaṭṭa’āt are the names of redactors. Zayd bin Thabit could have used the abbreviations to represent either one source of reader or for a reciter whose recitals were used to support the text over and above other variant readings. The readers were ranked in terms of reliability and importance; hence, the letters never violated the ranking that he gave them. Whether or not the names conceived of to fit the letters are the correct ones, we shall never be certain.
 Muḥammad bin ‘Īsā al-Tirmidhī. 1999. Sunan al-Tirmidhī (Vol. V) Ḥadīth No. 2910. Beirut: Dār I’ḥyā’ al-Turāth al-‘Arabī. p.175.
 Welch, op. cit., p.412.
 Massey, ‘Mystery Letters of the Qur’an’ op. cit., pp.497-501
 Ibid., p.499.