It was a bespoke build system but this aspect of it was similarly "wrong". IIRC pre-build commit, yes, was version-number related. It also gathered issues fixed in the release being built and updated change logs/release notes/upgrade info documentation automatically. IIRC the post-build commit helped confirm in the commit history that a particular build for release x.y.z was successful and the version number can now be incremented (occasionally it took multiple attempts for the release manager to build successfully). Of course, one could go through the tags but most of the developers liked seeing release management stuff in trunk/release branches.

I didn't mean to criticize SVN as being inherently incapable of reproducible builds (if anything that's harder to achieve with git, especially with hacks like I've listed above), but the act of cleaning up the SVN repos and preparing for migration to git where a lot of our old SVN (and RCS!) habits would be problematic, also seem like the same kind of housecleaning you'd need to prepare for reproducible builds.