I'm Pretty sure you're not referring to the serial number prefix. Just in case, it's date of manufacture.

1946-C
1947-D
1948-E
1949-F
1950-G
1951-H
1952-J
1953-K
1954-L
1955-M
1956-N
1957-P
1958-R
1959-S
1960-T
1961-U
1962-V
1963-W
1964-Y or Z
1965 AA
1966 AB
1967 AC
1968 AD



I am an N.R.A. Life Member,you should be to.