SCO_UNIX.420.rar contains "official" floppies that seems to be copied from
physical media. They are compressed with HD-Copy and unordered.

1.IMG contains the boot floppy, 2 and 3 the "filesystem" floppies, 4 the
"Foundation Set" floppy and 5 the first "BASE" floppy (SVR4 package). They
can be reordered by grepping 'PKG=<name>' and number obtained by grepping
'root.[0-9]+/'

Also, BASE2 is slightly corrupted (wrong checksum but file does not look
damaged), and DESKTOP3 floppy is missing from the set.

An unofficial build of USL UNIX Release 4.2 Version 1.0 can be found at
https://vetusware.com/download/USL%20System%20V%20r4.2%20v1.0%20full%20pkg%20set/?id=12786
and is repackaged here: https://archive.org/details/UNIXSystemVRelease4.21.0