Peak and Trough
Takes a number of signals and outputs the instantaneous high and low values of the set. See the Modcan site if you want a new one!
Positive & Negative Slew Generators
These were the ancestors of the Dual Universal Slope Generators.
Depending on when the module was built, I think this was one of two things: either a bi-directional electronic A-B switch, or a passive set of audio jacks of different kinds.
Pitch to Voltage Converter
This vintage beastie was apparently a fave among woodwind players for its accuracy and responsiveness. Serge didn't design the entire module; he incorporated some critical parts manufactured by Gentle Electric. This GE unfortunately went out of business, and the parts they made were potted, making their function very hard to reverse engineer. If there are any bright young E.E.'s out there willing to donate $50,000 worth of your time to this problem, the electronic music community will be eternally grateful.
Universal Equal Power Audio Processor