There is an excellent detailed discussion about the old Fuzz Face circuit on the web. It changes its sounds depending on the volume of the signal.
At low levels its has no distortion and produces muffled sounds. At higher levels it produces asymmetrical even-order distortion. At very high levels then its produces even and odd harmonics distortion.
It used old germanium transistors that have not been made for about 30 years (I still have some) and modern silicon transistors produce a different sound.