Revision A: Intensity pot in the middle, all black/white.
Revision B: Intensity pot to the right, Purple square on the bottom-left.
Revision C: Summer 2009, the purple square has been removed again. The rest is the same as the Rev. B.
Please also refer file attachments below.
All SCF versions have the same schematics and should act the same. On the Rev. B and C the circuits are made with SMT (surface mount technology). The old Rev. A was made with ordinary leaded components.
Some users claim that the old Rev. A sounds better - in any case, it's quite obvious that a more than 15 year old device would sound different from a new one, but whether that is the case is a matter of personal opinion.