My guess: Short time before or during the early years of WW1, say 1912 - 1915. After WW1, due to the political and economic situation in beaten Germany, up to about 1930, Germans could not import guns from abroad. As the German Mark (up to the staggering inflation of 1923) or the REntenmark/Reichsmark afterwards were not convertible, each Import had to be well founded and allowed for by the Reichsbank. So German gun catalogs up to 1930 listed some Belgian guns like FN Browning pistols and A5 shotguns, remarking "for export orders only". Further, the rare, proprietary 9x58R S&S cartridge was outmoded after 1918, replaced by the popular and more powerfull 9x57R.