The mount is a German EAW side mount, still made. See the EAW catalog:
http://www.eaw.de/eaw/webkatalog/drZf_21_AM_seitl_M.pdf
Warning:The base advertized for the Mannlicher-Schoenauer GK is intended for the post-1952 M-S rifles. It replaces the flat, dummy plate on the left side of receiver these late rifles come with. If you want to put such a mount on an older M-S, 03, 05, 08, 10, 24, 25, 1950, you have to fit one of the bases for the Swede M96 Carl Gustav or other Mauser.