I have seen exactly two of them since 1982, one of which I have. Mine is a regular production box. I suspect most of these went to schools back then, which would explain the difficulty of finding them today. The other one I saw was a prototype that Mike Dudeck had up for sale on his Tex*In Treasures website. He actually had it there for a couple of years, and sold it about a week before I inquired about it for purchase, as I had finally set aside the finds I needed to buy it--but I was just a little too late. I bought a partially assembled TI 128K memory board for the 99/4A instead.
These are actually a lot harder to find than the Video Controller sidecar boxes. . .