Big Larry and SCOSGT - Its a documented Ohio National Guard Shotgun. From the research that I done and from other forums - many of the "National Guard" 1897 Riot Guns didn't have the US, US Property, or Ordnance Bomb markings. Regarding the rear swivel - it was patented in 1940 - so while not a Winchester "Factory" Swivel its period correct. I don't know who installed it though.

I was told by the son of the original owner that while it was an OANG Shotgun it was probably used by the Ohio State Prison System.