I always disliked the use serial numbers to account for hardware features: PCB revisions are facts, serials are guesses. However, if you open up a 200k+ you're most likely to find an AES3-6 board, which is the last and more common board revision. I've recently modded an AES3-4 that had a serial 86xxx. I assume that AES3-5 starts at 120-150k. If this is correct the last two revisions (3-5 and 3-6) cover at least half of the existing Neo Geo cart systems. These are the revisions without sync out on the A/V port.
I can't tell anything about CD systems as I've never seen one.