Legend. This is an early model with non-linear potentiometers. Read up more about version differences on this great resource about Big Muff pedals:
http://www.kitrae.net/music/music_big_muff_tone_differences.html?fbclid=IwAR3dNfGSnFAcdK5btJ9RamJwu32_Qw9PxJRAyQAwnZioH2kJWtzXuBJp8dk
Quite heavily used condition, but in full working order.