There are a few very early cars that do not have the Shoulder harnesses. Ford waited until actually Jan 1st to include them. As a side note--for the standard Mustang (not CS), they dropped the louvered hood as a standard item for '68 (and back to the regular hood) to offset the cost of adding the Federally Required Shoulder harnesses; as per the bean counters.
So, you may see a '68 built before Jan 1st, 1968, with a louvered hood, and no shoulder harnesses. But! the begin build date for the CS was later than Jan 1st, (to be revealed at a later time) so there is a production time lag for the shoulder harnesses.
Which leads me to ask: who else has a January '68 GT/CS with NO shoulder harnesses? We just might nail the exact date again for a "production jump" through the Registry. Cars bucked on Dec. 28th, '67 might have not have included the shoulder belts, and were finished and out of the factory on say, Feb 15th (I can already see Jeff Speegle reading this one...).
In any case, look for the bolt holes up in the headliner--somewhat close to the coat hooks. A previous owner may have eliminated the shoulder harnesses, and covered the holes over with a new headliner. It's happened before (however, Dec '67 and earlier cars probably don't have the threaded bolt hole plates welded in the roof).
Paul N.