hm according to the info From : http://oldguns.net/sn_php/marlinlookup.php
I'd guess that it's pre-74 but that's just a guess. Given that your serial number
doesn't look like it would indicate a date, it looks to me like it would be pre-69
*edit*: well, in reading closer.. maybe not. I noticed what put in bold below...
MARLINS MADE 1948 and LATER
Marlin Model 336 and Models 39-A and 39-M manufactured from 1948 to 1968:
A one or two letter code was used to designate the year of manufacture.
All Marlins (except lever action) manufactured from 1962-1968
A two letter code was used to designate month and year of manufacture.
All Marlins manufactured from 1969 to 1990:
The first two digits of the serial number designate the year of manufacture, either as the last two digits of the year (in 1969-71) or as a number code
(1971 and later)..