a) Uniprocessor Free
b) Multiprocessor Free
c) Uniprocessor and Multiprocessor Checked (identical images, though two
    separate files exist) 

At least as of SP1 in Win2K, it no longer appears to be the case that UNI and Multiprocessor Checked builds are identical. Be careful setting up your WinDbg symbols!! 

There now appear to be eight separate builds of NTOS: 

1) Uniprocessor, non-PAE, Free-NTOSKRNL.EXE
2) Uniprocessor, PAE, Free-NTKRNLPA.EXE
3) Multiprocessor, non-PAE, Free-NTKRNLMP.EXE
4) Multiprocessor, PAE, Free-NTKRPAMP.EXE 

And then the checked versions of the above:  

5) Uniprocessor, non-PAE, Checked-NTOSKRNL.EXE
6) Uniprocessor, PAE, Checked-NTKRNLPA.EXE
7) Multiprocessor, non-PAE, Chcked-NTKRNLMP.EXE
8) Multiprocessor, PAE, Checked-NTKRPAMP.EXE