I compiled 2 versions of the test program on my Office 2003 system.

In one, I used the Office12\mso.dsl. In the other, I used the
Office11\mso.dll.

I then copied the prohects for each to my Office 2007 system.

I opened each project on the Office 2003 system using VB 6. In both cases,
the Office 2007 Office12 mso.dll was substituted

I then compiled both projects using VB 6 in the Office 2007 system, and
moved them back to the Office 2003 system. They both ran.

I guess that MSFT's goal is to permit such back and forth, as long as you do
not use features present in Office 2007 that are not in the earlier versions
of interest. This makes sense.

But the puzzle remains as to why some folkes have the Office12 mso.dll and
others do not.

one possibility is that one might have to have a compiler, such as VB 6 or,
some other language, and not just office.

If you just write VBA code within Office, the Office12 mso.dll is not listed
as a reference.

However, if you compile with, say, VB 6, the Office 12 mso is listed, but
the Office 11 mso.dll is NOT listed, thou you can browse and brute force
reference the Office 11 version. I ASSuME that the Office 12 mso could also
be referenced from within VBA, but I would not try it.