I have seen two Brit guns in this configuration. Monte Whitley had one for sale for a while. If I remember correctly his was 12x12x.360. Not sure if the Belg/UK gun currently on his site is the same gun. The other was a high end Birmingham gun (maybe an Osborne?) in 20x20x.22LR. Sort of a Lord of the manor's garden gun. Hard to have much of a rifle cal through the top rib.