|Posted: Tue Jan 22, 2019 10:31 pm Post subject:
|yes, it's the exacta mount, but only for telephoto lens.
I have a meyer 400mm f5.5 and it has the same mount.
Here is the explanation from http://www.wrotniak.net/photo/exakta:
Longer lenses with large apertures (180mm f/3.5 and larger) required wider lens throat than that provided by the standard mount which would cut off some light in the corners.
To accommodate these lenses, in 1953 Ihagee engineers introduced an upward-compatible version of the mount. It had two locking bayonet flanges: one internal (within the mount ring), compatible with "regular" lenses, and one external, protruding outside of the camera body. The latter is just ignored by "regular" lenses, while the "big" ones have a mechanical connection only for the external ring, with no mechanical parts entering the mount throat at all.