adeel, I have a copy of a book called The 336: A Collectors Reference On The Model 336 Marlin by Doug Murray published in 1983. According to it Marlin began using a letter prefix for serial numbers starting in 1947 and ended that practice in 1968. From 1969 to 1972 marlin used the last two digits of the year for the first two digits of the serial number. Example, serial number 69XXXXX for the year 1969, etc.
Starting in 1973, they changed again to subtracting the two digit year from 100. Example: 100 - 73 = 27. So for 1973 a serial number might be 27XXXXX. They continued this until 1983 which is when this book was published. My information stops there. However, I think it is likely that Marlin would have continued this practice until 1999 or 2000. After the change of the century, it would seem likely they would have to start a new serial number prefix or nomenclature to move forward.
So your serial number of 49,XXX would seem to be a rifle made after 2000 or newer because it obviously wasn't made in 100-49 => 1951, else it would have the letter prefix of "H" to the serial number. If my reasoning is correct, I would think Marlin would easily be able to identify the year of manufacture for you with a simple call since your gun is likely not that old. Where it typically gets dicey is for really old guns because records either weren't specific enough back then or were lost over time. But that is for rifles made way back around 1920 or so like my Marlin Model 39 was. It was definitely made in the 20's, but Marlin couldn't pinpoint a specific year for it.
