This lens is for camera which always has 28 frames visible. For some strange reason the part which is switching frame lines is made for 35 frames. Konica, Zeiss, Viogtlander, Leica using same geometry to bring frame lines. They are all M-mount cameras with compatible lenses.
To make this lens switching 28 frames, metal needs to be added. 35mm is less metal comparing to other frame-lines.