In case anyone's interested, looking at the schematic it seems:
Stage 1 is the final transistor stage of a Big Muff (i.e. the LPB-1).
Stage 2 is essentially the 2nd Big Muff stage.
Stage 3 is the first transistor stage of the Big Muff.
Stage 4 is where it becomes interesting... it's a Fuzz Face.
Apparently it's "Specifically designed to simulate “Imminent Amp Death”".
For me, it doesn't achieve that lofty aim, but instead reminds me of a Devi circuit, but with more clarity.
Here's a demo:
https://www.youtube.com/watch?v=1m1tBPDolYw