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 Fender-branded Prophecy basses adhere to the Fender Japan serial numbers scheme. These feature one letter (O, P and S have been seen so far) followed by six numbers. Officially, Oxxxxxx and Pxxxxxx are supposed to date to 1993-1994, Sxxxxxx to 1994-1995. By then Heartfield production had already ceased. It is not unlikely that Prophecy basses were produced for another few years (although not quite like the Elans and DRs that are currently still made under Fujigen's own FGN brand). These are not included in the Heartfield Guitars 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 conclude that serial numbers were assigned consecutively. As far as Fender serial number scheme instruments are concerned, this statement is even more valid.