Lookit what I found in my trunk..

I'm not sure if reverb units were available from any factory. This was a popular modification in the 50's and early 60's before FM stereo broadcasts and equipment were available. Basically a reverb unit would hook up to an AM radio with front and rear speakers and create sort of an "echo" effect (reverberation) and "expand" the sound. Sort of like when you're singing in the shower.
 
I know they were a dealer istalled option at least in 65; maybe other years. I wouldn't rule it out as a factory option either. The Mustang red book listed studio sonic reverb units but they may have used others.