In the words of Source Audio:

The True Springs reverb effects are based on a

mechanical design originated by the Hammond Organ Company in the late

1930s. This stark design uses a pair of input and output transducers to

inject an instruments dry signal into an actual set of spring coils and

capture the very distinct reverberations as the sound bounces back and

forth within the springs. Of course, Leo Fender adopted this design in

the 1960s with the some of his “blackface” amplifiers and a revolution

was born.