The "Duckbill" was originally developed by the US Navy Weapons Lab in China Lake California, for the Navy SEALs.
This shot spreader flattened the shot "cloud" into a long oval-shaped pattern, which made it easier to hit moving targets in the jungles of Vietnam.
The spreader was brazed on the muzzle of the Navy/SEAL standard shotgun, the Ithaca Model 37.
It worked by squeezing the shot column between the upper and lower prongs of the spreader, reshaping the shot pattern into an oval shaped pattern.
As I recall, the pattern was about 1-2 feet high, by something like 8 feet wide at 15 yards???.
The first models had an open, double pronged end which gave it the Duckbill name.
After it was found that extensive firing caused the open prong ends to spread apart, reducing the spreading effect, the later models were built with a closed end.
This fixed the spreading problem, but also slightly reduced the shape of the shot oval.
These Navy spreaders were designed ONLY for use with the Navy standard #4 buckshot loads, and could not be used with larger shot.
The SEALs gave them extensive use in Vietnam.
The first user, and possibly the most experienced shotgun user of all time was Navy SEAL Chief James "Patches" Watson.
Watson had the Navy Weapons Lab build him a custom Ithaca Model 37.
Watson's Ithaca had the first extended magazine ever on an Ithaca. Due to the barrel design, it was necessary to remount the barrel attaching assembly farther toward the muzzle, and fit a longer magazine tube.
After Ithaca saw a Navy Weapons Lab extended Model 37, they started producing their own version.
Watson had the Weapons Lab fit his Ithaca with a pistol grip-only stock with a ring on the bottom, to which he attached a cord and wore the gun around his neck.
Watson said that the pistol grip-only stock worked at the extremely short ranges typically found in Vietnam.
Watson used his "Baby" in two eventful tours of Vietnam, including one as a PRU and Phoenix program advisor with the Vietnamese.
In his book, Watson remarked that he'd been told since the war that #4 buckshot was ineffective, to which he stated "Nobody I shot ever complained".
He said that the combination of #4 buckshot and the Duckbill spreader made hitting running targets in the dens jungle at night easier.
He said the #4 shot preformed well on the small Viet Cong.
Eventually, China Lake stopped producing new spreaders, and the SEALs finally wore the remaining devices out.
No new spreaders were obtained by the SEALs due to the end of the Vietnam war and the specific conditions under which the SEALs used shotguns.
The spreader was and is entirely legal, unless there's some weird local ordnance.
In that respect, the spreader is no different than any other shotgun choke, except that instead of tightening shot patterns it reshapes the pattern.
There was a commercial company that made a version of the Navy Duckbill spreader, and there's a good write-up of it in an older Gun Digest Law Enforcement Weapons Digest.
This company is apparently out of business, and as far as I can tell, no new Duckbill spreaders are available.
The last I heard, the company was located in the Great Lakes region, but no one could get a response from them.