Manger PCB for tonebender 1.5

[jalmonsalmon]

 Hey all,
I was wondering if it is possible to use the mangler PCB for a tonebender MK 1.5 since after all that is a fuzz face circuit?
Besides the value changes on the caps for C1 - C3 and resistor R2, I was wondering exactly what C4 is doing on the mangler. I can see it is going out of the neg 9v and suspect it will not have any impact on the sound and just helps against an accidental reverse polarity - via that diode? 

Everything else seems to be in order though on that board    nice touch having R3 as the bias for Q2