KEY FUZZ

Key Fuzz is "based on the Mosrite Fuzz with a third transistor added to boost above unity gain" as built for Kurt by his guitar tech Ernie Bailey.

Volume on the left side, controls the output volume of the circuit. Higher settings will begin to overdrive your amplifier, which can add additional body and smoothness to the tone. Extra output available with the added makeup gain transistor.

Depth on the right side is essentially a blend control that mixes in a second transistor gain stage with the first. 

At the minimum setting, you're hearing the first transistor on its own -cleanish and fat. As you begin to blend in the second transistor's more voltage-starved sounds, and at max thinner, more aggressive saturation with a ripping top-end and rich girth. From a mild, sputtery fuzz to a harsh, "torn speaker" sound.

Piercing, clanky, vintage goodness that'll cut through any band mush.

9-18V DC center negative max

True Bypass 

Internal fizz adjustment is factory set to maximum to cut the harsh fizz, you can tweak it counterclockwise for more ear piercing shriek.

Like most vintage fuzz, it prefers to be first in chain after unbuffered guitar.