I have a Retina III C big, I got in 1966 as gift my aunts for a comb Xmas and high school graduation present. I still have it. Well by modern standards it is a finicky cameras, not only the interlock, setting the frame count, seeing the EI number, and setting the interlock, when I was 16 not a problem, not it is, As the light meter is not connected to the shutter, aperture interlock it takes time to set the camera up. Sort of sad as at one time it was good daily user. In collage I used it as a second body to my Pentax Spot and later Konica T.