Thank you, Guy and others who responded to my question.  Based on that information, it would seem reasonable to me to expect that a photograph of a STMFC with a repack stencil of 4 Sep 1951 would be dated sometime between that date and early December 1952, or did a more stringent rule apply before 1953 that would shorten the interval.  I am attempting to determine the approximate date of introduction of a particular British American Oil Company paint scheme.