Welcome to our forum 289Stang.
After seeing a reply from a post you made in another forum I discovered that your reverberator was produced by a company named Rotunda. That company disappeared in 1968 after Ford bought it. After that date Rotunda products were licenced under the FOMOCO name. The reverberator was supposed to give you an echo effect for your AM radio.
Thats all I could find about it and I know this answer won't be very usefull for you but maybe it will help someone to give you more information.
You can also try to get some help from some radio repair specialists.
Here is one for you here:
http://www.redwoodradio.com/car_radio_repair.htmI didn't find anything about it on Ebay but you can try the following links in the future:
http://motors.search.ebay.com/Rotunda_Vintage-Car-Truck-Parts_W0QQcatrefZC12QQfromZR8QQfsooZ1QQfsopZ1QQsacatZ10073http://motors.search.ebay.com/reverberator_Vintage-Car-Truck-Parts_W0QQcatrefZC12QQfromZR8QQfsooZ1QQfsopZ1QQsacatZ10073Good luck and if you have any picture of your reverberator and your Mustang post them here.