I haven't yet built a 275 but I have repaired them.
The yellow switche sends the input signal to the rear monitor out connectors. The black switches change the operation of the CV/control. In Near it operates as a typical dry/wet mix. In Recede CCW is no signal and as you rotate the control it fades in the dry and then a dry/wet mix and then to full wet at full CW. At center it appears to be about a 50% dry/wet mix.
I believe the original 275 was designed to be in a quad/surround setup. The voltage controlled mix outputs would be used in the front and the reverberation outputs would be used in the rear. I believe this is how it worked:
Near: The control simply adjusts the wet/dry mix for the front and the rear reverb is off for the first 1/3 of the control, peaks at 2/3 of the control, and then fades to off at maximum CW.
Recedes: The dry to the front is off for the first 1/4 and then increases to maximum at CW. The front reverb is off and peaks at 1/3 of the control and then fades to off at 2/3 of the control. The rear reverb operates the same as off for the first 1/3 of the control, peaks at 2/3 of the control, and then fades to off at maximum CW.
The two reverb tanks are Accutronics/Belton 1BC2A1B. Decoding the number results in a 9.25" long x 3.375" wide x 1.313" high tank with a 150 ohm input impedance, 10,000 ohm output impedance, and a medium decay (1.75 to 3.0 sec). The input and output connectors are grounded with no spring lock and the tank is mounted vertically with the connectors facing up.
The Equalized Output is out of phase with the input and unity gain.
The Monitor Output is out of phase with the input and unity gain.
The CV Output when set to Near dry is in phase with the input and unity gain.
The CV Output when set to Near wet and the Reverberated Only Output are the same amplitude which vary with frequency. Phase of course changes with frequency due to the wave propagation time through the spring. There are a number of peak resonances. At 100 Hz the amplitude of the wet is just slightly less than unity.
At 200 Hz the amplitude of the wet is just slightly greater than unity.
By 500 Hz the amplitude of the wet has begun to decrease but still sounds reasonable.
At 1 KHz the amplitude of the wet increases somewhat.
At 2 KHz and above the amplitude of the wet is minimal.