I recently got a C1. It had thick felt in the groove, thick enough that the holder and GG frame were held away from the metal. The felt at the hinge end of the holder had been mashed crooked, holding the GG and holder at an angle. Since the holder and frame are shaped differently, the angle they were held away from the metal was different. So the focus was different. And the holder was at angle enough to let light in along the sides. Maybe why the seller got rid of the camera. I removed the felt and put in strips of black velvet. The velvet is just thick enough so that the fibers stick above the plane of the metal, but thin enough so that the holder and GG frame push the fibers down and the holder and GG frame contact the metal. I used black silicone to glue the velvet in. I masked the edge of the groove with masking tape so the silicone didn't get all over, used a little brush to put silicone in the groove, and also on the back of the velvet strips. Set the strips in place and held it with a holder till it cured. I cut a step in the strip ends so they interlock, stopping any light trying to get in at the corners. I also put a narrow strip at the bottom of the groove the holder ridge fits in, that might not be neccessary. I thought of using double sticky tape (permenance?) or contact cement (hard to reposition). Don't use silicone if you want to do any painting, a trace of silicone prevents paint from sticking.
