Graduated filters can be used on rangefinder or twin lens cameras, although their use is complicated by the fact that you cannot see the filter effect through the lens before exposure. The way to get around this problem is to mark your filter holder with alignment marks that match the filters position on your particular lens / camera. This can be done by trial and error, or more accurately by opening the back of the empty camera, opening the shutter on a bulb exposure and checking the filter position through the lens – you may have to focus onto a piece of ground glass or similar material – you can then mark the filter holder to show when the filter is ½ way down, 1/3 or 2/3 (some people just mark a halfway point). This only needs to be done once for each lens and will make using your filter system on rangefinder cameras much easier.
LEE Filters now offer calibrated rangefinder blades for the standard system to help with this, or of course the super compact Sev5n system.