These mounts were developed during WW1 by the Danzig arsenal for sniper use. The solid rings were soldered to the steel scope tubes. Apparently a gunsmith adapted a surplus scope and mount to the Mannlicher –Schoenauer Stutzen after the Great War. Quite hopeless to find spare parts now, 100 years later. Here are pics from Dieter Storz's book "Gewehr und Karabiner 98", Vienna 2006.