Dunlop GT-OD M193 Overdrive Guitar Effect Pedal for sale online | eBay
The MXR classic overdrive pedal has a hidden switch inside that changes to the Zakk Wylde overdrive. I've seen people mod their own switch for it, but I just drilled a hole
This is interesting. MXR Classic Overdrive GCM66 | Fender Stratocaster Guitar Forum