Also the reference and blog posts are useful: https://code.kx.com/v2/
It’s not as easy to get information as other languages, but the mailing list is super helpful and all the questions I’ve asked have been replied to by paid Kx (the people that make Q) employees:
This is the arthurese micro APL implementation, which inspired the J implementation - although IIRC the design for J was already well on its way at the time (and indeed, Iverson did not consider J a new language, but rather an improved APL dialect, so you could say the design goes back to 1956...)
It likely also formed a draft for A/A+ which Arthur did at Morgan Stanley (open sourced at aplusdev.org, but unmaintained and rotting theese days) - which themselves led to the creation of K.