User blog comment:Silver-Haired Seireitou/Bleach Reveals: Light Novels and Databook Translations/@comment-18812574-20180428114607/@comment-18812574-20180429134215

Yep. I just read the section where the role of the Tsunayashiro is mentioned:

"“If it can be said that the role of the Shihōin clan also known as the Tenshiheisōban, is to handle armaments rumoured to have been bestowed upon them by the Soul King – then the role of Tsunayashiros is overseeing the past itself. The Great Spirit Book Gallery is where all intelligence is gathered, and before that collection method was established, the Tsunayashiro clan were the caretakers responsible for documenting the entirety of history.”

“They were the Caretakers?”

“With regard to the aspect of ancient history, even the study of historic monuments require the permission of that lot, and the publication of certain documents are subject to strict censorship. I’ve heard that in some cases, historians would go missing too and in no small numbers.”"

So they were both the record keepers prior to the Great Spirit Library and the most powerful clan (allegedly).