It is Gustav Kersten's "Double Greener crossbolt", invented in 1899, but merely protected for 6 years by aDRGM = protected design #123783. As Kersten was a military armorer,as such a government employee, he was not allowed to take a patent for his invention then. As Kersten was stationed in Strassburg, again German from 1871 to 1918, since then French Strasbourg, the breeching was also known as "Strassburger Verschluss". Though not often seen on side by sides and drillings, the Kersten lockup became virtually a standard for continental European over-unders. All the classic Merkel, Suhl, and many Ferlach made over-unders, shotguns, combinations and double rifles, feature Kersten's lockup.