There exists somewhere a website that breaks down every marking on carbine parts, but I can't find it at the moment. I definitely would not worry about the letters, most of the parts on carbines were made by subcontractors and assembled looking like alphabet soup. I'd ask for a clear picture of the markings on the top of the barrel. My inland is serial number 5137xxx with inland barrel dated 3-44. i have letters all over the parts on mine. I'll keep looking for the website that breaks everything down, if I find it i'll pm you. Here's a link to a pic of mine along with some of its brothers:
http://www.flickr.com/photos/32289199@N03/3843158014/
Mine wears a WWII stock with all post war updates. My stock doesn't have any marking but is a very nice piece of wood.
GregM