THis is a standard European made Under & Over ( or O&U) gun, one barrel smooth, the other rifled. The Rifled Bore is usually for one of the rimmed "Drilling" cartridges ( 7x57R, 8x57JRS, other "proprietary design " cartridges)
Commonly used in Central and Eastern Europe for general forest hunting ( Deer, Wild Boar, large Birds, Aquatics etc.)
"Cape Gun" Usually Double barrelled Shotgun, with last part of barrel/s rifled to use "Paradox" cartridges ( a brass shotshell with a large slug, usually conical.); English development.
Adds to versatility, and as a back-up to the more common double rifle or magazine rifle. "Cape" refers to Cape province, South Africa.
Also refers to Cape Buffalo, the idea being you want two independent actions for reliability rather than a magazine repeater in case of jams or misfires when The Horned Death comes out of the brush and charges you- you don't have time to eject a dud or work the action for the next shot, you just pull the second trigger.
Generally "Cape Guns" are the large-caliber doubles meant for launching two big single projectiles, while a drilling or combination gun generally has a rifle cartridge barrel and a shotgun cartridge barrel.
Neither kind is particularly uncommon... It's just they usually cost as much as a used car and don't sit on the rack at a gunshop for very long. I'm astounded the one you linked to didn't sell, but hey, bad economy.
This is a Bockbuechsflinte or O/U
a cape gun is a combination side by side gun with a rifled barrel on the left and a shotgun barrel on the rigth side made in britain and belgium for the south african market (the cape). common chambering's for cape guns are 303 brit, 577/450 MH and 450 Musket.
with the rifled barrel on the rigth side and the shootgun barrel on the left side it would be german/austrian Buechsflinte. this was also made in other country's like sweden from firms like Husqvarna or in Belgium for the gemran market.
every combination gun with one barrel over the under is a "Bock".
If I had the $$ I'd buy it myself as it is a C&R and has some of the best workmanship and engraving seen in a long time. Wonder if it was one of those that the germans surrendered to the GIs at the end of WWII. My dad says everyone was so scared/sick of war every old family "treasure" was dumped in the streets.