It depends on where you got the source code. If it’s the mozilla-central or mozilla-unified Mercurial repository, your SM version likely corresponds to Firefox Nightly (currently version 106, soon 107).
You can use ./mach run -- --help | grep Version to see the version number. For the Nightly branch I currently get this:
$ ./mach run -- --help | grep Version
Version: JavaScript-C106.0a1
Did you ever figure out how to build a different version, though? I’m currently trying to determine those steps so that I can try to build something older than v63.