This is the basic circuit for several versions of the DOD OD250, the MXR Distortion +, the Ross Distortion, and the DeArmond Square Wave, just to name a few. The only difference between all these is differing component values (and sometimes not even that). The component names from the above schematic follow those in the layout drawing below.
For a full BOM for 6 different versions of this circuit, download the linked file below. In addition to the BOM, it contains scaled transfers for etching your own circuit board and a scaled drilling template for building the effect into a Hammond 1590B enclosure.