I think they're codes relating to the print runs or something to do with the printing.

Interestingly I just looked at the back of a few QF passes I have handy. One of them has: A B C D E F G H I J K L M N O P Q R S T U V W.

Another one starts a E...

Actually now I come to think of it it reminds me of a previous life when I worked in the production/printing area of a major daily newspaper. Anyway they used to print various letter codes (eg: A , B etc) each time they changed the plates .. or rather, as a version control. So the first set of papers printed would be an A, then if they changed something, edited something, added a late sports score, updated an article(whatever), the next run might have a B on it and so on. That way you could tell which version you had (I think some papers use series of stars, and/or print stuff like "FINAL"). It was basically a version control system.

The other reason to do it may simply to be as a test of the letter rendering of the print process. I dunno. I'm not a printer. I'm sure it's got something to do with it though

(a new way to learn the alphabet when bored on long flights? )