Verified working. This is a modified BOSS Blues Driver by Keeley. This rare piece of equipment is known for being used by John Mayer. As the name "Phat Mod" suggests (also playing on "FAT" for thick and good sound), the bass is boosted when the switch is turned on, resulting in a dense and very cool distortion sound. It also retains the core clean sound of the Blues Driver, making it a very user-friendly pedal. Due to the small number of units and its high level of completion, few people sell them, making it a very valuable piece of equipment nowadays.