Date codes of manufacture for Heckler and Koch firearms are as follows:

 A  B  C  D  E  F  G  H  I  K
 0   1   2  3   4  5  6  7  8  9

The letters correspond to the numbers directly below them and those numbers are the last two numbers in a year. For example, a pistol with the date code KI marked on the slide would indicate that that pistol was made in 1998, unless of course it's already 2098 (in which case we probably would be shooting at each other with phasers set on stun). H&K started counting again from 00, or rather AA when the fake new millennium began.