Yeah this has been doing my head in trying to find some logic to it. They might be throwing a letter in front of all the coneheads to separate them from the other 3 seekers. For instance thrust can't use 11T as it's already used.
N could stand for Nibbio which is what Ramjet was called in the Italian release of G1. Or it could be like 8X, 10A and 13B and not really have a meaning.