At a local show I picked up a Security Six stainless 6 inch barrel, unfired in the original box with owners manual. Now for the question. I checked the serial number chart on Ruger's website. It's a 154 series but the serial number sequence doesn't match the chart. The Ruger 154 series starts with 633xxx. This revolver is 154-060xx. None of the series listed have a zero after the series number except the first year (1972). This something to write Ruger about or do any of you forum members have an explanation?