Matt The Modulator's Circuit Bent Gristleism:
• Extended the pitch range from slightly higher to a lot LOT Lower.
• Optical eye (LDR) to control the pitch with light (with on off switch).
• 12db-per octave filter with Cut-off and resonance pots (which can oscillate at high settings).
• Low-Pass or Band-Pass filter select switch.
More of Matt's modifications can be seen on his website: