Modified variation, only available to purchasers of the one board run, less than 30 people got it. Officially unobtainium. If you have a PCB purchased second hand, the person it was purchased from is the only person who should provide you with that info, just like a pico PCB. If you purchased a board from FCS, your invoice would get you a replacement schematic. Put any Pico related schematics up here, or link to one, it'll be taken down as per Pico user agreement. Working with THAT app note is your best bet. It is apparently borderline stable, with layout being critical.
FWIW, I don't ever use the attack/release mode on any of my Pico. If you are after a similar Pico compression sound with more control, you'll get the control but not the sound. The stock mode is really where I think the Pico shines. A/R is based on modified Peak, I never use Peak mode either.