It may be the month. If it does not have a code it is pre 1949. A-1949 B-1950 C-1951 D-1952 E-1953 F-1954 G-1955 H-1956 I-1957 J-1958 K-1959 L-1960 M-1961 N-1962 P-1963 R-1964 S-1965 T-1966 U-1967 V-1968 W-1969 X-1970 These codes can be found in the Standard Catalog of Firearms. The I says it was made in 1957.
Help to Identify Stevens Shotgun. From the serial number it has. They went back into production basically as the previous version with the J. Stevens Arms Co. In 1886, the company was reorganized and incorporated as J. Stevens Arms & Tool. Most Stevens long guns did not have serial numbers, so it is impossible to. 620 was introduced in 1927, and manufactured into the 1950s. Stevens Arms Co. Serial number info is long gone and even the Savage Arms Corp. Historian can't tell you.