The number is placed there automatically by some old version control system (probably SCCS or RCS) and is incremented on every commit. The commits were likely just adding a new year to the copyright line.
Anecdotally (wasn't there at the time):
sccs was more common everywhere before RCS because sccs was part of all unix (e.g including BSD UNIX).. it was encumbered as part of the UCB USL suit and rcs use grew after that, then cvs, then svn, then git
