OS version detection

How does base 8.12.4 detect the windows version?

I am trying to use hfslip with the fdvfileset.  I am slipstreaming Windows 2000 with the hotfixes suggested by the_guy.

Base is complaining that OS:WHY-DOESNT-THIS-HAVE-A-VALUE.

fdv attempted to make a compatibility change which I suspect doesn't work. [I thought it worked when I first
tried it but I can't get it to work now.]

fdv has suggested that the driverpacks determine the windows version using Majorversion udner [SetupData].

I would like to try to understand why.  What is the difference between the virgin sources and the slipstreamed sources (after hfslip and fdvfiles) that the OS version can not be detected?  Would it be possible to allow a manual over-ride?

Thanks.