The Minolta SR-mount was the bayonet mounting system used in all 35mm SLR cameras made by Minolta with interchangeable manual focusing lenses. Several iterations of the mounting were produced over the decades, and as a result, the mount itself was sometimes referred to by the name of the corresponding lens generation (f.e. "MC", "MD" or "X-600") instead.
MD - In 1977, the XD series introduced an additional tab on the lens which reported the smallest available aperture (f/16, f/22, or f/32) to the camera to accomplish shutter priority automatic exposure in S mode on the Minolta XD-7 (XD-11 in the U.S.A.), and later in the P (programmed automatic exposure) mode on the X-700. The proper use of this feature meant that the lenses had to be set to their smallest aperture. In 1981, MD lenses included a minimum aperture lock that prevented the aperture ring from accidentally being moved.
Material from http://en.wikipedia.org/wiki/Minolta_SR-mount
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