This is the rear base for a German claw mount, also sometimes called a Suhl claw mount because it was developed in the city of Suhl during the golden age of German rifle craftmanship (late 1800's / early 1900's). The front base is missing. It should be in the dovetail on the barrel ring, and it is supposed to be permanently fixed. In most cases, a traditional Suhl mount involved silver soldering the bases, in addition the dovetails. Occasionally a set screw replaced the silver soldering.
To install the scope, the front scope ring's, or rail clamp's, two claws are supposed to swing down into the front base from the top, and the rear scope ring or rail clamp's two claws are supposed to insert into the rear base where the spring loaded bar snaps over them.
To remove the scope, the two "ears" of the rear base are pulled back, this frees the rear scope ring's, or rail clamp's, two claws, and the scope can be swung up and out.
This has long been considered the Rolls Royce of detachable mounts and it is absolutely reliable when properly installed, but installation is long and delicate as the front claws must be stoned with absolute precision to match the base. This explains why it is now regrettably mostly something from the past, but New England Custom Gun in Claremont, NH typically has some parts, and a few old hands like Paul Chapman at Griffin & Howe in Hackettstown, NJ know how to use them.
A well executed claw mount is very tight, and is typically greased. This one is on a custom made Dumoulin .338 Win Mag stutzen carbine. A perfectly adapted rifle for deep-woods silent stalking of massive European boars.