The FK is the mark of Frank Krack who was the Chief of Small Arms Inspection at Rock Island Arsenal from the Fall of 1941 to the Summer of 1946. So your shotgun was rearsenaled in that time frame.

I tend to agree that it was a riot gun that has been converted to a trench gun. But I'm not certain RIA did the conversion. I don't recall seeing an arsenal riot to trench gun conversion where they left the magazine end cap stop and locking pin on the gun. These were usually removed. And I believe the bayonet adaptor does not fit properly because it is too far forward on the barrel. They sit farther back on the WWII trench guns. Maybe it can be pulled back flush with the magazine cap unless the transverse grooves for the bayonet adaptor were not cut properly.