Serial number is FK503101.

Frame built in Nov 1985 as a 1986 model year bike.

Per T-Mar:

SERIAL NUMBERS 1980-1987*:

Serial number format WXYZZZZZ where;

W = a letter, presumably indicating a Fuji facility or contractor. To date, samples have included F, I , J, K & S.
X = a letter, indicating the calendar month of manufacturer (A = January, B = February, C = March, etc)
Y = a number, indicating the last digit of the calendar year of manufacture (0 = 1980, 1 = 1981, 2 = 1982, etc.)
ZZZZZ = a five digit number sequence, probably indicating frame number during month

Example: FB125349 indicates the 25,349th frame manufactured during February 1981.