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 a unique production number within that month.
For example: A89123 would have been manufactured in January of 1989, and assigned unique number 123.
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.
Some Heartfield-related instruments, so far some Prophecy basses, adhere to the Fender Japan serial numbers scheme, which can be checked out at http://www.fenderjapan.co.jp/faq/serial.htm. These are not included in the Heartfield Central serial numbers database.
Although it is tempting if you look at the similarities between some near-the-same-serial-number runs of the same type of instruments, it might not be correct to include that serial numbers were assigned consecutively.