Many original pickups of the older SG’s, SF’s, SX’s and SC’s have a number stamped on the back.
Look at this SF 500 pickup:
The first two digits on the back indicate the year in the Showa period. The Showa period is the period of Japanese history corresponding to the reign of the Showa Emperor, Hirohito, from December 25, 1926, through January 7, 1989.
Year 50 = 1975, year 60 is 1985 etc.
The pickup number starts with 53, making it a 1978 pickup.
The serial of this SF 500 is 003384. Looking at the “SG number chart” that also indicates 1978.