This seems to be the rear base of a "Triebel'sche Einhakmontage" = Triebel's claw mount, made from about 1910 to the 1930s by Albert Wilhwlm Triebel, Schillingstr. 8, Suhl ( one of 50 Triebels active in the German guntrade) for the trade. Here are two lousy pics of the rear "foot" of such a mount. The rectangular bosses fit into the front slots and position the scope. The two hooks are connected by round stems and screws to the springloaded, checkered crosspiece at the front. To release the scope, you grip the scope, push back the crosspiece with thumb and forefinger and lift the scope off the base. Maybe the easiest to use quick release nount ever n´made.