My best guess is it uses a Canon FD mount but have not tested. Here's why I think it's a Canon FD: 

• It has a breech mount. Canon FL/FD lenses are among the most well-known breech-mount lenses.
• The location of the linkages matches those on Canon FD mount lenses. Note there are two "pins" on the FD mount. One to "auto" control the aperture. The other to communicate the aperture position for aperture priority.
• The lens has screws that match those on known Canon FL/FD breech mounts.