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
2004 Jim Dunlop MXR ZW-44 Zakk Wylde Overdrive Pedal Serial# | Lot #81195 | Heritage Auctions
MXR CSP016 ZW-44 Zakk Wylde Overdrive | Reverb
MXR Classic Overdrive - Flip a Switch to Make It a Zakk Wylde ZW44! : 5 Steps - Instructables