Trek makes a reverb unit which can be added to most Hammond Consoles.
See 
http://trekii.com/rv-1d.html
Electro Tone used to make a similar reverb unit known as the OrganMate. Schematics and other information on
the Electro Tone OrganMate reverb unit can be found at http://www.geocities.com/OrganMate/.
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