You've come to the right place! The creator of this game, "Alex" on the forums, will probably chime in soonish.
In the meantime, I can tell you what I know: an author named Matthew Skail has published some 3rd party content on DrivethruRPG in the form of several custom classes. I bought a few, and I pulled one up to look at the back. I see that at the tail end of the OGL designation he listed, among other things, copyrights and dates for the core ACKs book and the player's companion. In addition, after that he has a page dedicated to the ADVENTURER CONQUEROR KING PRODUCT IDENTITY LICENSE Version 1.0 so I'm guessing that's going to be where your bread and butter is.
edit: googling didn't produce an official source but somebody appears to have reproduced the text online: https://campaignwiki.org/wiki/BeremAndBeyond/OGL