I have a kingdom of fighters, mages, halflings, and elves. My fighters and mages attain max rank without a problem because of their training buildings, but the other two races' buildings don't take them that high. When I looked at the unit screen of an elf and a hobbit at the highest level their buildings get them, they had letters after the level number. What do those indicate?

These are 1st edition d&d rules. Humans don't have a level cap, but other races (like elves and halflings) have them. So, what they did in this game is a sort of level up over the cap with letters.