Hammond oil spring reverb system
JMI Vox Echo-Reverberation Unit -"Cliff Richard Reverb" (1962 - 1968) A Look "Under the Hood"
I found some patents that disclose the use of piezoelectric transducers with spring reverb/delay systems.
US 2375004 Training apparatus
US 3136853 Music enhancing systems
US 3189686 Transducer and mounting for mechanical delay lines
US 3246264 Transducer assembly for reverberative delay lines