My Model of 1905 4th change has none of the usual markings on the barrel top--Pat'd dates, S&W address. And it looks newer than the reat of the gun. Replacement bbl? Yet the SN stamped on the bottom of the barrel under the extractor rod matches the SNs on the handle bottom and cylinder, except for a "B" stamped before the bbl SN. I see and feel a slight radius decrease of the bbl diameter starting just behind the front sight blade. Since the revolver seems to have been reblued LONG ago, might the usual markings on the barrel top have been sanded down and off by S&W?