Garmin AT support line.
Dial 1-800-525-6726. At the first menu, press "1", then enter extension 3991 followed by the "#" key. You may get any of these knowledgeable guys on the phone - - John Hendrickson, Terry Clair, or Gary Read. They can tell you anything you want to know.
The letters you asked about usually indicate engineering design changes (simple modifications) to improve performance and reliability.