Nick, If my memory serves me correctly, ALL SL & R series cameras need a circular polarizer to meter properly. With that sais, I'll go out on a limb and say that I think that the early 50 & 35's that use a series 6, actually have a 48mm filter thread in the lens, so that all you need to do is het a 48mm-to-55mm step-up ring and you can use the latest leitz filters. You can also use the excellent HAMA of Germany, rectangular lens hoods which work all the way down to 24mm focal length without any vignetting!! Hope that helps Art Tafil natstek@xxxxxxx