Dear Hiddenhunter;
I seem to remember something about this from a gun collectors association magazine they delivered to my home, no maybe it was Shotgun News.
I would suggest that you write to your local office of the Bureau of Alcohol Firearms and Tobacco and tell them that you are thinking of acquiring a very short barreled shotgun and was hoping one of their folks could help you acquire one. I would bet they keep a list of all companies that manufacture such things.
I am sure they would be interested in talking to you. I seem to remember a newspaper story not long ago from Idaho where BAFT was involved in something like this.
You might want to be careful though as some such firearms could be illegal to own depending on legal details.