Tatramajjhattatā • Equanimity • In the middle of all that Friends, could anybody direct me to some suttas of the pâli canon about Tatramajjhattatā, the developed form of the equanimous abiding 'in the middle of all that'.
To the best of my knowledge, this specific mental factor is mentioned in the Abhiddhamma and the Visuddhimagga, but I was unable to find it referred to in the Pali Canon.

Many thanks for your much appreciated help, with Mettā
Upekkhā word is used in 10 contexts of suttanta with 10 meaning, which were analysed by sāriputta-aggasāvaka, then concluded to 4 sabhāva:


One refers to vedanā (upekkhā-feeling).
One refers to viriya (effort).
Two refer to paññā (understanding).
Six refer to tatramajjhattatā (equality/parity/counterbalance).


So, when sāriputta try to explain each upekkhā, he used the synonym words to explain them, like we saw in the dictionary, nowadays, such as in KN. Jūlaniddesa, Udayamāṇavakapañhāniddeso:


  [446]    Upekkhāsatisaṃsuddhanti   upekkhāti   yā   catutthajjhāne
  upekkhā   upekkhanā   ajjhupekkhanā   cittasamatho   cittappasādatā  majjhattatā   cittassa. 


That is the original tatramajjhattatā in pāli cannon.

Why commentary concluded 10 meaning to 4 sabhāva?

Because of the main topic of each contexts, which each upekkhā word appearing. For the example in SN Mahāvagga, First Ānandasutta:


  [1393]   Yasmiṃ   samaye   ānanda   bhikkhu  tathā  samāhitaṃ  cittaṃ
  sādhukaṃ    ajjhupekkhitā    hoti   upekkhāsambojjhaṅgo   tasmiṃ   samaye
  bhikkhuno   āraddho   hoti   upekkhāsambojjhaṅgaṃ   tasmiṃ   samaye  bhikkhu
  Bhāveti   upekkhāsambojjhaṅgo   tasmiṃ   samaye  bhikkhuno  bhāvanāpāripūriṃ
  gacchati.


You can see sama-word and ajjhupekkhā-word, which appeared in Udayamāṇavakapañhāniddeso like tataramajjhupekkhā, explaining upekkhāsambojjhaṅgaṃ. So, commentary concluded them as the same type upekkhā, tattaramajjhupekkhā.