The Quark Vintage Overdrive is a clone of the Maxon OD-880 Overdrive / Soft Distortion, originally released in 1976.
This pedal was huge in Japan, being the favored overdrive pedal of Char, one of the country’s most popular guitarists, who—like the pedal—never really got noticed out of Japan. It was reissued a few times, notably in the mid-90’s as well as more recently.
It’s an interesting circuit from a historical perspective, being sort of a “transitional species” between the Distortion+ and the Boss OD-1 OverDrive. It added the input and output buffers and changed the Drive potentiometer to a flat op-amp boost instead of a gain modifier that also affected the bass tone, as in the Dist+. The Boss OD-1 is essentially an OD-880 with feedback clipping diodes instead of diode-to-ground clipping. This diode arrangement turned out to be a very big deal and made way for the Tube Screamer a couple of years later.
The Quark is a direct clone of the OD-880, but with a few additions. A clipping toggle allows switching between three different sets of diodes. An EQ switch engages a second mode with increased gain and more bass. And since the OD-880 lacks a tone control, a Treble switch selects between three different post-clipping hi-cut modes.