Far as I can tell from asking JD, and experimenting with the circuit, the 'contour' knob was a variable resistor to ground off of the "Big Muff" style tone stack - 'replacing' R8. It definitely borrows from Jack's tweaking ideas here, the 'body' or 'presence' control http://www.muzique.com/misc/presence.pdf
, the latter name I do find a misnomer, but it is what it is.
This is a very useful mod for some. I like the 'full mids' with the stock 82Kish range on the BSIAB, including 56K, 100k, etc. As the designer Ed notes himself, most amps are already designed and/or EQ's for mid scoop...so scooping more mids is not necessarily a good idea. Many people find the stock Big Muff Tone stack with a lower resistor (I'll throw 22k out there) which scoops the mids a fair amount means things get lost in a live setting, YMMV. Mid scoop is good in moderation rather than excess for most tones...but it depends on what you want, the rest of the circuit, etc.
When the BSIAB or anything gets that gainy, things naturally sounds brighter and chunky on the low end, downplaying mids (meaning it seems
more mid-scooped). The full, VH hot-rodded marshall type mids into the inevitably mid-scooped, set-bright fender amp are what it the pedal was designed around, also, bear in mind. I like the midrange girth the circuit has even with lower gain settings. So I personally would not want to scoop much more mids...but in theory, it's a great option, and can make it sound 'heavier' and/or more 'modern;' forgive the cliches. Likewise with really low gain, being able to dial down the mids could give you more perceived clarity for bluesy single notes and neck pickups, etc.
Try breadboarding it with a various resistor values (82K, 56K, 47K, 33k, 22K, 10K, 4k etc) and see how you feel about it, or what range of values you find usable. Or, you can just make it a 100K pot, drill the hole, be done with it...my question is...if you go with the 100K pot, should it be linear or audio taper? My gut and bad memory say audio taper, but you gotta use your own ears.