The file frmts/l1bdataset.cpp appears to have stagnated and an update is required to support NOAA-18 and MetOp?-A.
NOAA-18 has been around for a few years, as has MetOp?. First, these (and future) satellites need to be recognised. Second, the file format has changed slightly a few times.
It is important to check the file format version number stored in the file since files in both old and new format will exist.
The NOAA KLM Users Guide is the definitive reference. There is an Amendments page which will help you find out what has changed.
http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/index.htm
However there a couple of helpful documents available here:
http://www.sat.dundee.ac.uk/AVHRR1bFormat.doc
http://www.sat.dundee.ac.uk/AVHRR1bDiffs.doc
The first may be out of date now but may be easy to read and search.
The second is a short description of the main changes made to add support for NOAA-N, NOAA-Nprime and MetOp?. You may want to read this document first.
(The documents were obtained from a now-defunct NOAA FTP site)
This page may also contain some of the required numbers:
http://www.sat.dundee.ac.uk/satids.html