There are 2 letter date codes.
A B C D E F G H I K
0 1 2 3 4 5 6 7 8 9
So - AE is 2004 (04). KG would be 1996 (96). Understand
All the P99s had a closed accessory rail and the small bump under the triiger THRU 2003. Then, in 2004, they changed the frame. It had an open rail under the barrel, but still had the short magazine release. They also reshaped the trigger guard. Then, in 2005, the changed the frame again. It now has a longer magazine release lever, and the trigger guard was just slightly reshaped again. All P99s made since 2005 have this newest variation to the frame.
I have a 2004 compact P99 A/S, and a 2006 fullsize P99 A/S