I know the concept has been around for longer in the US, but FYI here is the earliest article I have found, without looking too hard, on converting shotguns to double rifles.
Enjoy!

Gun Digest, 47th Annual Edition 1993. Anyone here know of any earlier articles? If so, please post details. The author claims his gun was built in the '70s, and I assume the BSS conversions started pre-1990 as well. There must be some nostalgia out there in print!