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The 1.5 came after the Mark 2 Tone bender and they were made in a very limited number making this one of the rarest (if not THE rarest) Tonebenders.
This "Tonebender" has much more in common with the fuzzface's of the time featuring just two transistors.
The best way to hear this pedal is to compare it against our Dallas Aribitor Fuzz face and the other Tonebenders on TonePedia.
This will allow you to experience for yourself exactly what the difference is and what makes the 1.5 extra special ... One word ... "Sizzle"