Everyone reading this has already heard of the Pedersen device. The nifty little conversion bolt that let you swap out the bolt on a M1903 Springfield, stick a magazine in the side of the modified action and start cranking off .30 pistol rounds in semi auto. Raking enemy trench lines with more firepower than your bolt action as you charged at a German machine gun from across non man’s land.
Supposedly there are less than 100 of them still left in the world thanks to the US government not wanting them to fall in the wrong hands. The “wrong hands”, as always, being you.
Anyhoo. The ammo for it is probably even more rare and someone managed to run across some. The thumbnail image and the image below being a box of it. This shared by one of the fellows in the vintage gun group. He had no idea what it was.