The duckbill was developed in the early 60's at the Naval base at China Lake, then moved to Crane Weapon Station. The prototype was listed on gunsinternational a couple of years ago with the blueprints and a picture of the China Lake quarterdeck inlay. IIRC it started at $5K. I grew up with the 37 and do a lot of searches for it, stumbled across that one.
It was developed for the then newly formed SEALs. Sr. Chief "Patches" Watson had the first and tested it in Nam. The SEALs standardized on the Ithaca 37 and he had China Lake create a pistol grip for his 37 to go with the duckbill.
In his book Walking Point he said the enemy never complained after being shot with 0000 buckshot and the duckbill.
The effectiveness of the pattern was brought up on another website a year or so ago. It really isn't optimal for birds or clays. But for spreading laterally has some merit but not many have the need for that.
We had a 37 in the locker on a Gator Freighter I was on. Everyone knew to keep their hands off it because it was "mine". When the Marines switched to the 9mm in the mid 80's they came on board and took the 37, M14's and .45's. Sad day.
The Mossberg 500 and Rem 870 replaced most of the shotguns from what I saw until I retired in 95. Few years later the Benelli M4 started to replace those from what my friends who were still active told me.
