The Horizon Amp Overdrive is a recreation of the Catalinbread Dirty Little Secret Mk I, a unique drive pedal that resembles a Marshall amp stack in its response and tone.
The DLS Mk I was the first of three versions, and is often considered the best—or at least, different enough that subsequent versions should have been released as new pedals. They share some DNA in common, but they have very different tonestacks and tweakability.
The DLS circuit uses cascaded mu-amps as a foundation. There are no op-amps or clipping diodes. The clipping signal is generated entirely by the JFETs, giving it a different flavor than diode-clipping-based overdrives.