He has his own subversion to keep track of versions. Currently, he seems to be at revision 1706. As you can see on the nightlies download page, not every revision is released to work as stable.
He has been working on a gdbm database backend, which is a huge modification to the program. This is why he hasn’t released any new revisions and probably won’t for a while.
Long story short, the revision number increases when he checks in code changes to the subversion server. I guess the next release will be 1708 or up.