There were a lot of different ots shotguns used, Mostly sawed off pumps. There were also a lot of Japanese A-5s, also shortened.
The land warfare center had a lot of experimental SGs, e.g. full Auto 1100s, Duckbill spreaders,suppressed, and One belt feed I've seen. Also a cut down Win. 97 with six inch Bbl. and folding grips, a three shot.
There were also some four and six Bbl. break open models prototyped.
The British used A-5s cut down with #4 buck to great effect. When they were in Malaysia.
On the folding stock for the 870. Here is the rest of the story. The Secret Service wanted a pistolgrip 870. Due to red tape and bugeting there had to be a production ots piece that they could get. In a roundabout way they got the Marine Guards to submit need for a folding stock gun to be used to protect embassadors. This was enough reason for our elected officials to approve for the budget R&D and aquisition of these. The SS designed and approved the design. You will notice that the folding stock is easy to remove and does look like an afterthought. All you have to do to get money from the politions is to tell them it is for thier own protection.
On the folding stock 870, I have used one for several years. The folding stock is just for storage ease. I have modified mine by machining an aluminum block to fit inside the stamped sides. It contains a mercury recoil reduer. This adds a little weight and stops the rattle. I've shrink wrapped a six" piece of closed cell foam around the stock over the reducer and done the same with the "short" buttstock. This lowered the recoil covered the sharp edges and stopped the lose stamped steel rattles. Also I use a bungee cord sling. It is reasonably comfortable to shoot with the LE low recoil loads.
Cheer,
ts