Usually on the bottom tang, underneath the lever. Some later 39a's have it on the top tang, behind the hammer.
I found on the web site below, that a 336 Marlin with the serial number that begins with a "P", was manufactured in 1957. http://oldguns.net/cgi-bin/f2f/f2f.pl?http://oldguns.net/q&a1_05.htm
I HAVE THE SAME QUESTION. I WANT TO SELL MY GUNS, DUE TO A DISABILITY, BUT I CAN'T FIND THE SERIAL NUMBER ON MY FOX MODEL B. ANY HELP?
Check the 2nd valve casing.
If manufactured prior to 1969, it doesn't have one.
Should date to 1983. First two digits (17) from 2,000=1983
Your Marlin model 30AS was made by Marlin in the year 1995 with the serial number that you provided.
With the serial number that you have supplied,your Marlin model 60 was made in the year 1976.
Your Marlin model 60 was made in the year 1996 with the serial number provided.
Your marlin model 39A was made in the year 1980,with the serial number that you have supplied.
Your serial number indicates that your Marlin model 995 was made in the year 1984.
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.