Also, within a model family the same serial number prefix may be used to produce a variety of different models, all in the same block of serial numbers. There are occasions when blocks of serial numbers have been manufactured out of sequence, sometimes years later. Ruger does not necessarily produce firearms in serial number order. It can be used to determine the approximate year your Ruger firearm was It is not necessarily the very first serial number shipped, but This number should be used as a point of reference The above chart shows the approximate first serial number shipped for Ruger New Model Single Six Serial Number History