Going through the safe tonight, and fondling a Single Six that has had too little use lately, I noticed a small detail that needs some clarification, for my own curiosity. There is a D&T'd, plugged hole in the receiver, centered port-to-starboard and 1.5" forward (center-to-center) of the rear sight elevation/mounting screw. At some point in the past, this revolver appears to have worn a scope base, and so I'm curious as to if this is a factory hole. The serial # is 560xxx. For the life of me I cannot recall whether or not I have seen this screw hole on other Single Sixes.