I was actually incorrect by a couple of years in my 2009 post to this thread. A Model 1897 with a serial number in the 732xxx range would have been made in 1920 (at least that is when the receiver had its serial number applied). These Denver PD trench guns were either Military guns (acquired from the Military or National Guard), or Military overruns or commercial trench guns acquired directly from Winchester.

These is a photo on page 64 of Canfield's Military Shotgun book taken in 1921 which shows members of the Denver PD with 1897 trench guns. They still had their bayonet adaptors on them back then.