Different manufacturers use different methods, but these are standard.
The alpha characters could desiginate the model number (Even your initials as part of the serial number would desiginate the "model" number).
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
Life Member NRA
Life Member NAHC
Former President of the ECPT (Eifel Combat Pistol Team)