The 9 m/m smooth bore garden gun was available in the U.K. A Parker Hale catalogue circa 1930,refers to the gun as the,"Three Bore [9 m/m] Garden gun.It was sold as a low cost single shot bolt action gun. I believe the cartridges were imported into the U.K from Europe illustrations appear to be C.F.
Webley and Scott also sold a;3 Bore Bolt action garden gun, but this gun used R.F shells.

Roy Hebbes