Why no triple alpha letters?

[drumminor2nd]

So I have some questions about a used-stamped 5906 with an odd KMF prefix serial number, and I think a letter could help answer out some questions. As the gun came from a family member's estate, I'm not looking to sell it and it cost basically nothing -- so a letter would be a nice addition to the gun for the collection.

So I went to start the process of getting a gun lettered, and I saw this notice on the website:

PLEASE NOTE: At this time we are unable to letter Smith & Wesson firearms with “triple-alpha-numeric” serial numbers that date from 1982 to about 2013.

Does anyone know why that is the case? Were they able to letter them before? When they might be able to letter them in the future? (it's the weekend, otherwise I'd bug them directly)