Fenwick serial number date codes run as follows based on the first letter in the serial number:
So your "N" series rod would have been from the 1975-76 period.


A-1960-61
B-1961-62
C-1962-63
D-1963-64
E-1964-65
F-1965-66
G-1966-67
H-1968-69
I-1968-69
J-1969-70
K-1971-73
L-1973-74
M-1974-75
N-1975-76
P 1976-77
R 1977-78
S 1978-79