Matt Orsborn tells me that "In the Dazhidu lun [T 1509] there is a nice discussion on 明咒 and 咒".
A translation and study of this discussion would be another very useful project for a grad student in Buddhist Studies to tackle. Where the Heart Sutra has 咒 (mantra?), T 223 has 明咒 = vidyā in Pañc. (cf my 2017 paper on the "epithets").
Again, this kind of study is beyond my capabilities but I could make good use of the results.
@jayarava
From what I've read, mantras weren't a thing yet in the 7th century. (in China, that is) as they hadn't been imported pet from Tibet. Ah, but do I have a citation for that? No, I don't and don't remember where I read it. It may have even been you. 😀
@AndyLowry I've certainly commented on this issue. Mantra started appearing earlier (tho decontextualised) and then were introduced from India in 651 by Atikuṭa, the first proper Tantrika to arrive.
Atikuṭa translated the dhāraṇī that ended up in the Heart Sutra giving us a terminus pro quem for its composition.
@AndyLowry No warranty is implied... 🤣