The first shotgun I ever fired was a trench gun in MP School (1973 @ Ft. Gordon, GA). I wish I could remember what make it was.
I fired a Wicnhester 1200 at FT Gordon in 69. Could be what you shot also. It was probably not a "trench gun" as the ones I saw at Gordon and in Vietnam did not have the heat shield or bayonet lug. Just a riot gun.