OK here is the first of the Verney Carron slider info, although there are several others, I think this will erase any doubts the Verney contributed to the evolution of todays modern sliding breech shotgun. In 1925 Verney was granted an English patent for his improvements in the sliding breech lock up. GB283928. In 1928, another patent was issued, but for the double button lock up as found on the Stopvis (GB306649) which clearly states it is for single and double barrels. Then in 1930 Verney was back to the traditional top key when he was granted a patent in Germany (DE492485), besides any patents that were issued in France and Belgium. I've seen a Verney Carron darne type, but never a Stopvis double, but no doubt some were made.