The Original 35b & 50b had quite large rear sight units that clamped to the barrel near the breach or they could be unclamped & moved to the rear of the cylinder so the apperture/eye cup unit could be fitted. The clamping screw has a series of small parallel grooves in it. These grooves rotate as the screw rotates & all the ones I have seen have a red coloured material on the end of the screw & around it. The material is slightly gooey, not set hard like paint, its not dry & crumbly like perrished rubber. I wondered what it is & what it was I wouldn't have thought it was intended as a thread locking material as the sight was designed to be moved between the two points at the users digression or whim. I'm assuming it was originally a pad of some sort to prevent the sight marking the rails & would have been formed on the screw end or perhaps be a push fit in the small recess under the sight, but that wouldnt be very secure. Theres not much space in there. Do other owners have similar red material under the sight? Have you ever replaced this with a modern material? Just curious.