The “General” and “Admiral” insignia have no letters. Being the two highest ranks, it is possible the reason for this is that they no longer strive to “live up to” old war heroes - they’re already held in high esteem.
• The “Colonel” insignia translates as “KAPLAN”, referring to the Imperial warlord. Sadly not much is known about him yet.
• The “Major” insignia translates as “TARKIN” - referring to Wilhuff Tarkin, the Grand Moff of the first Death Star who died of his own stubbornness. He was portrayed by Peter Cushing in Episode IV.
• The “Captain” insignia translates as “DILLON”, which does not refer to a character, but rather to Glyn Dillon, who was co-costume designer for The Force Awakens!