Following on from some discussions about SCM history on here over the weekend I was poking around in the history of things too.
The original 1986 CVS¹ is quite interesting from a historical prospective. Given that the implementation is just a bunch of scripts on top of RCS you can see how it plumbed together really easily.
There is a note¹ on Grune's site, that explains the relationship between the scripts and the system people would more likely recognise as CVS(assuming you're old enough to recognise it at all).
¹ https://ftp.sunet.se/mirror/archive/ftp.sunet.se/pub/usenet/...
² https://dickgrune.com/Programs/CVS.orig/CVS_BB_and_GNU
Edit: And I guess you might not recognise the archives in the first footnote either. They're shar files which can be extracted using the files as a script or more simply with unshar from sharutils³.