Hello Jim.  Welcome to the Winchester Collectors Forum.

I saw the photos of your shotgun over on the Gunboards forum.  The P in an oval on your barrel indicates it is a Winchester replacement barrel that was installed by someone other than the factory.  It could have been replaced by the Military or been replaced after the gun was released from the Military.   The lined out FULL choke mark indicates it was cut down to riot length.  Can you confirm the length of the barrel for us?