Buildbot is not upgraded on slaves at the same time as on masters. So it has its own version history, represented in http://hg.mozilla.org/build/buildbot/ by the slaves branch. This is *not* merged with the default or production branches.
Each deployed version of Buildbot is tagged, and the filename on the slaves corresponds to the tag. So, for example, SLAVES_0_8_0 is installed in /tools/buildbot-0.8.0 on slaves. We use 'rN' suffixes for patches to buildbot that are not from upstream, so for example the next version will be SLAVES_0_8_0R1', installed in /tools/buildbot-0.8.0R1.