In addition to the 4 knobs on the top of the pedal,
there is a 5th located on the side of the pedal next to
the input jack.  This is actually a 5-way switch.  It is
the Input Capacitor Selector Switch.  This switch sets
the Fuzz Master apart from any other fuzz out there.  
It gives the Fuzz Master true sound contour and a
tonal spectrum unlike any other fuzz.  By switching
the input capacitors, you can go from that "woofy"
ultra- bassy sound of the original Arbeiter Fuzz Face
to a nasty nasal and everything in between and still
keep that warm tone that has made the fuzz circuit
legendary.  
Then there is the "hybrid" switch.  When in
germanium mode it engages two NOS Mullard
OC75 germanium transistors for that pure vintage
fuzz sound.  Switching it to hybrid mode replaces one
of the transistors with a high-gain BC214 silicon
transistor to add more sustain, punch, and fizzle to the
fuzz.
The little black rubber stopper located next to the
input jack is the easy-access internal trimpot.  
Tweeking your fuzz has never been so easy.  Just pull
the stopper out and stick a small screwdriver inside.  
No need to remove the baseplate and flip it
upside-down.