Re: "S7" Not sure if your looking for physical location or schematic location...According to the control box schematic on my 1991 "S7" is identified as "Start Disc Sw". On schematic it is located right above the S1 Table (lower right corner of schematic diagram).
J16A <<------P59A16----"S7"----P55F16>> J16B
Furthermore, S7 is referenced on pg 4-38 of TM5-61115-584--12 under "DC Control System":
"The circuit between 25-27 of the master switch applies battery
voltage to the field flash resistor through diode (CR2).
This action applies field flashing current to the
exciter field. It sled applies power through diode
(CR1), which prevents starting set when the battery
polarity is reversed, and the closed contacts of the
start disconnect switch (S7) to the crank relay coil
(K3). When the crank, relay contacts close, a circuit
is completed to the starter solenoid (K6), which
closes and starts the cranking motor."
So...is "S7" the switch identified as "item 4" within the Starter Lockout Switch and Gear Assembly" on pg 4-32?