According to The Rokkor Files website, ". . . the XD Series had introduced a new Shutter Priority exposure mode, requiring that the lens be set at its minimum aperture for correct operation. It is noted that both the XD Series and X-700 cameras will, in fact, work in Shutter Priority mode and Program mode respectively without being set to minimum aperture. However, for full operation, including correct viewfinder displays, the activation of the MD coupling lug is required.", and, further, that the MD aperture lock, introduced on the third generation MD lenses and also called the Program lock, was, "designed to ensure that when shooting in program mode on the X-700 the lens would not switch from minimum aperture", facilitating proper exposures on the X-700 by beginning users - one could just lock the lens, set the camera in Program mode, and shoot. [Copied for educational purposes only, underline emphasis mine].