On Sinaron f/4.5 DB lenses there may be a little red screw that limits the lens to f/5.6, so that it will play nice with the older f/5.6 Sinar shutter. I have that old shutter, and a Sinaron f/4.5 lens without the screw... anyone know where I can get one, or what thread size it has so I can substitute a normal screw?