1966 Reverberator

I have been doing a lot of looking, but have found little information on the reverberators that came with some Mustangs. I have a reverberator that is in very poor condition mounted in the trunk of my 66. I would like to rebuild it somehow, but have found almost nothing on the internet about the units. Only that they were there.

Does anyone have any information (tech specs, pics, resto steps, etc) on these units, or at least where to look? Any help would be greatly appreciated! Also, were these units installed at the factory, or were they added as Dealer options?

It apparently used to be covered in some kind of cardboard, which has long since deteriorated. On the inside is a bar supported by springs that, from what I have heard, was suppossed to cause an echo effect for the AM radio.

I just wish that I could find something on them since I want to take the car back to as original condition as I can and would love to have that functioning and looking original. Thanks kendall for your info as well...I did not know that it was a Rotunda/FoMoCo accessory. At least that gives me a little more to go on.