Here is a picture
It is an additional VCO which is fed from the internal VCO then mixed back into the filter, it is capable of a number of different effects depending on the settings, see the youtube demos for the kinds of sounds it can make. Certainly for techno or experimental music I feel it takes the Monotribe to a whole new level, and I am proud to say that a few of these have ended up in the hands of some techno legends. It is a wild little mod for sure and I won't be directly replacing it, although I do have some new and different mods coming soon around summer time.
Price is £55 plus shipping £5 UK £9 rest of world, to order hit the email link at top right of page.