Heartfield serial numbers consist of one letter followed by 5 numbers.
The letter stands for the month in which the guitar was manufactured: A = January, B = February, C = March, D = April, E = May, F = June, G = July, H = August, I = September, J = October, K = November and L = December.
The following two numbers represent the year: 89 = 1989, 90 = 1990, 91 = 1991, 92 = 1992 and 93 = 1993.
The final three numbers indicate the sequence of that month.
For example: A89123 would be the 123rd guitar manufactured in January of 1989.
It should be noted that some Heartfield instruments use a different dating system (primarily Talons). This dating system seems to be similar to that of Ibanez at the time: The letter is still the month of manufacture (A = January, B = February, etc.), the following digit is the year (1 = 1991, 2 = 1992, etc.) and the rest indicates the serial number within that month.