On Marlin-branded Model 60 rifles, the first two digits of the serial number indicate the year of manufacture. For example, if 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. Therefore your Model 60 was manufactured in
1978.
.
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.
Happy shooting! I bought mine new in 1993 and have never regretted it. I've probably put 2000 rounds through her since then.