The letters were used as a quality control method. The sabers were made by injection molding plastic into steel molds. The molds had multiple cavities. Each cavity was marked with a different letter. This allowed a quality control employee to identify a damaged mold when a saber wasn't formed correctly . This was a more important step when the saber molds were initially created to manufacture double telescoping sabers which were more prone to malformation during the manufacturing process. In fact this is likely one of the major reasons why the sabers were ultimately switched to single telescoping sabers. When the switch happened the original DT moulds were retooled forthe single telescoping sabers and the QC letters on the hilt remained. Presumably once the original moulds wore out,newer moulds were created specifically for the single telescoping sabers and were created without the QC letters.
Sabers can have letters on either side of the hilt because the moulds likely had 2 sides that mirrored each other.