If you can wait until this afternoon, I can ask the product manager of Aurifil (he's coming to give a presentation at LQS). As far as I know, Aurifil doesn't/hasn't used letters to refer to their threads. If I had to guess, I'd guess those letters probably have something to do with either the distributor or vendor. This is what #2000 thread should look like, per Aurifil:
http://www.aurifil.com/wp-content/up...0-1030x689.jpg