I may not be right on this, but, I believe the lettering identifies which background to use for the shot.
Most of the spare backgrounds that I have purchased come with markings on them, mostly numerical, however, there are some letter ones as well. Ocassionally, a background will actually tell you the episode in which is was used, but for the most part, there will generally be regular numbers that will appear on the right upper portion of any background. But, generally speaking, you'll just see regular old numbers on the upper right hand portion of the background.
The only letters I've noticed so far on my backgrounds are "C", as in "cut 224". It will generally be abbreviated on the background as "C.224" or something simliar. The corresponding douga will also feature matching annotation as well.
For example, here is a background used in Cardcaptor Sakura episode 51 and its matching genga.
Background only:
[img]"
http://img.villagephotos.com/p/2002-8/4 ... height=850[/img]
Genga w/ BG:
[img]"
http://img.villagephotos.com/p/2002-8/4 ... height=850[/img]
...Gah...can't get the images to show up on the page. Sorry...
