These are completely different publications but on the same subject. In the GPO project, for 2 years we worked through all the records for monographs. Some of these are things weren't found the first time we looked for them, so there are still records in various stages of searching. But we are basically through with the bulk of the monographs. Now we are beginning to work on serial records.
Serials have the same title and are published more than once.
A serial could be an annual report, a journal, or a newsletter. There are some exceptions, of course. These are generally things that when they were first published, were thought to be a monograph and had a cutter after the colon. When they were published again, a year was slashed on to the first call number. Serial records have also been set up for monographs which are revised on a frequent basis.