This in general is derived from the port number used by the responder to the initial SYN packet. If the unidentified traffic is coincident with an ftp connection between the two hosts, then it is reported as ftpdata-XXXX instead. See below for discussion of local and remote hosts. Thus you may trust one of the byte counts, and the other is probably equal to or just a bit below the final byte count, though it could be much below if the connection persisted half-open for a long time.
N indicates the connection was with a neighbor site.
See below for discussion of neighbor sites. As distributed, this function matches the networks belonging to the Lawrence Berkeley Laboratory, where the script was developed. As distributed, this function matches the networks belonging to the University of California at Berkeley, since the link between LBL and UCB is very fast, and I wanted to be able to flag traffic between the two sites as possibly atypical for wide-area traffic. If you don't care about discriminating between neighbor and non-neighbor sites, you can just have the function always return 0. If neither is set, then hosts have their full IP address in the reduced output.