That gun was most likely manufactured in 1979. Marlin Firearms manufactured between 1946 and 1968 have a letter that precedes the serial number. Firearms manufactured between 1969 and 1972 are dated by the first two numbers in the serial number. i.e. 1969 firearms begin with 69, 1970 with 70, etc. After 1972 you must subtract the first two digits of the serial number from 100. In your case, the serial number begins with 21. 100 - 21 = 79. Or, 1979. You can get more details here: http://www.gundealersonline.com/firearmID_Marlin.asp
Your Marlin model 30AS was made by Marlin in the year 1995 with the serial number that you provided.
Your Marlin model 60 was made in the year 1996 with the serial number provided.
With the serial number that you have supplied,your Marlin model 60 was made in the year 1976.
Your serial number indicates that your Marlin model 995 was made in the year 1984.
Your marlin model 39A was made in the year 1980,with the serial number that you have supplied.
marlin 22 model 39A serial number Y6410
Your serial number indicates that your Marlin model 60 rifle was made in the year 1973.
Your serial number indicates that your Marlin model 336 was made in the year 1971.
Your serial number indicates that your Marlin model 1894 rifle was made in the year 1982.
Your serial number indicates that your Marlin model 39A was made in the year 1981.
Your serial number indicates that your Marlin model 60 was made in the year 1985.
Your Marlin model 25MN was made in the year 1990,with the serial number that you have provided.