Without providing the serial number there is no way to tell. The Model 66 began production in 1971. Call S&W with the serial number and they will tell you when it left the factory.
04
@1969
1977
No serial number given = no answer.
1979
80-81
You must call S&W
You will have to call S&W
1986
You must call S&W
Call S&W with the serial number and they will tell you.
You will have to call S&W for the date.