It's a 1955 since it has the 'N' prefix. I would guess that you could get $350 for it in good shape at auction. But I'm just guessing. I have a 1956 model with the 'P' prefix.
The letter U which is a prefix to your serial number is used to date the production of your Marlin lever action rifle.In this case the letter U indicates that your rifle was made by marlin in the time frame of 1960-august of 1961.
See the link below. You will need the serial number, including any letter prefix.
The H prefix to your serial number indicates that your marlin model 336A was made by marlin in 1951.
The W prefix to your serial number indicates that your marlin 336 was made in 1963.
The W prefix to your serial number dates your marlin model 336 to 1963.
The K prefix to your serial number indicates that your marlin model 336RC(regular carbine) was made by marlin in 1953.
Your marlin model 39A with the prefix letter S to the serial number,dates your marlin to the years 1958-1959.
With the letter J prefix to your serial number,this Indicates that your Marlin rifle was made in the year 1952.
The H prefix to your serial number indicates that your Marlin model 39A was made in the year 1951.
The V prefix to the serial number dates your marlin to 1961-aug.1962.
The W prefix to your serial number indicates that your Marlin model 336 was made in the year 1963.
J prefix is 1952 date of manufacture