Back in the day, synthesizers weren't the all-in-one integrated packages you see today. The equipment consisted of several modules wired together, each with their own function. Programming these devices was done by connecting patch cords between these modules. Moog was the father of the modular synthesizer design later adopted by other companies such as ARP.

Modules used in Moog Modular Systems:

901     VCO / LFO
901a    VCO Driver
901b    VCO
901c    VCO Output
901d    VCO Output
902     VCA
903     White Noise Generator
903a    White / Pink Noise Generator
904     VCF
904a    Low-Pass VCF (the famous patented one)
904b    High-Pass VCF
904c    Filter Coupler
905     Spring Reverb 
907     Filter Bank
907a    Fixed Filter Bank
909     Power Supply
911     Envelope Generator
911a    Delayed Trigger
912     Envelope Follower
914     Extended Fixed Filter Bank
921     VCO
921a    VCO Driver
921b    VCO
923     Noise Generator / High- & Low-Pass VCF
928     Sample/Hold
930     Power Supply
950     49-Note Keyboard
950a    ?
950b    Scale Programmer
952     49-Note Duophonic Keyboard
952a    Duophonic Keyboard
955     Ribbon Controller
956     Ribbon controller
958     Pedal Controller
959     Joystick Controller
960     Sequencer
961     Sequencer Interface
961CP   Sequencer Interface Panel
962     Sequential Switch
984     4x4 Matrix Mixer
991     VCF / Attenuator
992     CV
993     Trigger / Envelope
994     Jack Multiples (2/4)
995     Attenuator
995a    Reversible Attenuator
1120    Foot Pedal
1123    Sample/Hold
1130    Drum Controller
1131    Percussion Controller
1150    Ribbon Controller
1528    Sample/Hold
1584    Mixer
1617    36-band String Filter
1630    Frequency Shifter by Harald Bode
1634    Pitch-to-Voltage Converter
6401    Ring Modulator
CP1     CV/Trigger Outputs
CP2     CV/Trigger Outputs / Filters
CP3     4x1 Mixer
CP3a    Mixer
CP4     CV/Trigger Outputs / Attenuator
CP4a    CV/Trigger Outputs
CP5     CV/Trigger Outputs
CP6     CV/Trigger Outputs
CP7     CV/Trigger Outputs / Multiples
CP8     Power Switch
CP8a    Power Switch
CP9     Power Switch
CP11    Mixer / Triggers / Outputs
CP35    Attenuator
DVCO-1  Dual VCO
SF-1    String Filter
I created this list by combining several more incomplete ones found on the web. I can't say this one is final and complete, though. /msg me if you know of more units that exist.