The mount comes from Ernst Apel, Würzburg (EAW) http://www.eaw.de/de/produkte/aufschub-aufkippmontagen/komplettmontagen-aufschub.html

This kind of mount seems to be quite popular in Scandinavia, judging from the number of Scandinavian rifles seen with three threaded holes on the left side of the receiver where the base once was.