Age of Marlins
I took this off another forum talking about Marlin 60's (the tube feed version of the 70) but the date is right on for my 70 (a coast to coast model 42).
On Marlin-branded Model 60 rifles, the first two digits of the serial number indicate the year of manufacture. For example, on the rifle shown in the photo above, the serial number starts with 18... The 18 is significant, for subtracting it from 2000 gives the date of manufacture for the rifle, 1982. Likewise, a serial number starting with 17 would be a rifle manufactured in 1983.
This method has not been uniformly followed. It is valid only from 1971 through 1999. Prior to 1971, the first two digits indicated the year of manufacture. For example, a rifle with a serial number starting with 70... would have been made in 1970. Starting in 2000, this same old method has returned; the first two digits once more indicate the year of manufacture directly, with no subtraction required. [2]
Recently it seems Marlin has changed their methods once again as one example purchased new in 2006 has a serial number starting with 94, the exact way to find the date seems unclear, though it is possible they are using a "subtract from 2100" method.