Does anybody know the screw/thread size of the mounting screw on the 686.? i just bought a B-square scope mount and the screws included are either larger or smaller than the original mounting screw for the rear sight. called S&W to ask for the thread size and i brought the screw to the fastener shop. S&W parts guy said don't know and the fastener shop said don't have it from, try S&W. any help would be appreciated, thx.