“I believe that source control was not widely prevalent until 1999.”
Uh, no. RCS (and its ancestor SCCS) has been around since the 1970s:
Anybody doing development anywhere near a Unix box was using SCCS or, (after 1980 or so) RCS. There were many home-grown variants of it (DSEE, SourceSafe, MPW Projector). In nearly 30 years of professional software development I’ve only witnessed one shop that didn’t use it (and I fixed that in a hurry when I joined…)