One of these is, that Ajahn Sao Kantasīlo, then abbot at Wat Liap, taught Ajahn Mun to repeat the phrase "Buddho" as the initial step to meditation. Frankly, the mantra-like repetition of 'Buddho', introduced into the Thai Forest tradition by one of its most influencing founders sounds to my ears very "pre-modern Mahā Nikāya".
One can read an extensive description of this in Ajahn Muns biography:
This brings us to the next factoid that is repeated endlessly, namely that Ajahn Sao Kantasīlo was formally Dhammayut-ordained (apocryphical formulated as: "He rejected the textual orientation of the Dhammayut movement, and set out to bring the dhamma into actual practice.") and strongly influenced in his meditation teachings by Somdet Phra Vanarat Buddhasiri (1806-1891), himself one of the founders of the Dhammayut movement and tutor to Prince Mongkut and -- next factoid -- in turn influenced by orthodox Mon monks whose philosophical reach led to the founding of the Sri Lankan Forest Tradition as well.p. 4:
When Ãcariya Mun first began practicing vipassanã at Ãcariya Sao’s center, he meditated constantly, internally repeating the word “buddho”, the recollection of the Buddha, as he preferred this preparatory Dhamma theme above all others. In the beginning, he failed to experience the degree of calm and happiness that he expected, which caused him to doubt whether he was practicing correctly. Despite his doubt he didn’t flag in his persistent use of the word “buddho”, and eventually his heart developed a certain measure of calm.
Unluckily, I was able to find only very little on the specific teachings of Somdet Phra Vanarat Buddhasiri, besides that he has written the Caturarakka Kammathana, eg The Four Objects of Meditation That Give Protection. Even though it is such a seminal work. I have not even be able to find a version of the full text in Thai, but there is another work by Somdet Phra Vanarat Buddhasiri, collected by Ven. Tan Chao Khun Upālī, the Saṅkhitt'ovād: Exhortations in Brief.
Only problem is: no mantra-like repetition of any phrases is taught, the only approximation would be the meditation on the qualities of the Buddha. This is a far, far cry from how Ajahn Maha Bua advises to begin meditation practice, p.494:
I would be very grateful if somebody involved with the Thai Forest tradition could enlighten me to the doctrinal source of this meditation method (not the actual method itself, I already saw the discussion of Buddho, here: viewtopic.php?f=41&t=2552&start=225), and provide me with the text of the Caturarakka Kammathana (in Thai would be ok).buddho: Supremely enlightened. A traditional epithet for the Buddha, buddho is a preparatory meditation-word (parikamma) that is repeated mentally while reflecting on the Buddha’s special qualities. In its simplest form, one focuses attention exclusively on the repetition of “buddho”, continuously thinking the word “buddho” while in meditation. One should simply be aware of each repetition of “buddho, buddho, buddho” to the exclusion of all else. Once it becomes continuous, this simple repetition will produce results of peace and calm in the heart.