The Poseidon Silicon Fuzz is based on the Catalinbread Katzenkönig, originally released in 2015.
The Katzenkönig (also spelled Katzenkonig or Katzenkoenig) is a mash-up of the silicon Tone Bender circuit (input and transistor gain) with the back half of a RAT Distortion (clipping diodes, tone and output volume recovery). But as with most Catalinbread circuits, even if the source material is familiar, the result doesn’t sound much like anything else.
Interestingly, Catalinbread published the schematic of the pedal in the manual under a Creative Commons license. Catalinbread has always been very friendly to the DIY community, and they reasoned that since much of the development of the silicon Tone Bender and the tracing and analysis of the RAT had come from DIYers, they wanted to give something back.
The Poseidon is a direct adaptation from this schematic, with one addition: a diode clipping switch has been added to go between the stock set of diodes and two other options with higher thresholds for more headroom and volume.