Most of my knowledge is 1970 specific, so if I am wrong, someone please correct me....
3123 - 3 - last digit of year, 1973. 123, 123rd day of 1973.
Edit;
I think I got that bass ackwards, this makes more sense for your car;
312 - 312th day 3 - 1973 (1974 model year)
Barry