My favorite spring reverb is up for sale. I’ll miss this one and it is the last and best sounding of my spring reverbs. With AM-4 amplifiers, of course!

Working great and has the following convenience and safety modifications:

• 220V wiring brought up to modern specifications. This thing had some pretty dangerous and non-code-compliant power wiring when it was produced. All of this has been sorted out and made much safer and more reliable. See the photos where I’m pointing to the insulation and modifications.
• XLR and 1/4” input and output jacks added. The terminal strip is still active if you wish to use that. 
• Tired capacitors replaced (note the blue ones that I’m pointing to in the photos)

It's a variable decay, single channel spring reverb in a rack unit, dating from around 1972. Quad Eight described it as a new and patented approach to mechanical reverberation, with a clean, transparent sound that compares favorably with other types of reverb.

The novelty is in how the springs are used. There are four separate transmission lines, each giving a different initial delay. The full delay pattern is built up after 55 milliseconds, and the reverb then runs in four continuous trains of multiples of those delay times. That's different from a typical two-spring tank.