1946 to 1953: Transitional I Frame, Improved I frame 1st Version, Improved I frame 2nd Version
1953 to 1958: Model of 1953 New I Frame (Pre Models 30 and 31).
1958 to 1961: Models 30 and 31 I frames
1961 to end of production: models 30-1 and 31-1 J frames. Serial numbers after mid 1968 had a letter prefix.