Looking at similar 67 A and C code cars like yours (PS only) I found single alternator pulleys.
I wondered also if there were any RPO's (regular production options) available for any 289 engine Mustangs WITH PS
which could have come with a dual pulley drive on the alternator and water pump pulleys (since I recalled seeing this arrangement, but wasn't sure where).
Since I DO NOT have a 68 Assembly manual to refference, I'll use the 1967 Osborn Engine Equipment Assembly Manual to source my information (sort of what I suggested in my first reply to this thread).
Maybe somebody with a 68 Assembly manual can confirm these results since I know there are changes going into the 68 model year at least in the brackets and Thermactor System.
I have found for the 1967 model year (since the O.P. didn't specify exactly) that Ford did not intend this "dual pulley design" to be used on Mustangs (or Cougars) WITH power steering but did offer it on other carlines WITH power steering (such as Falcon, Comet, Fairlane) so the pulley system is "out there" for the dual pulley alternator, for use on a 289 engine WITH power steering, but not "correct" for a Mustang.
*see 1967 Osborn Engine Assembly Manual references:
* w/AC, w/o P/S (dated 4/29/66, page 75)
* w/o AC, w/o PS, w/Thermactor, w/std. & extra cooling AND 55 amp RPO alt. (dated 11/7/66
**, page 89)
* w/AC, w/o PS, w/Thermactor (dated 5/27/66, page 101)
and lastly, the O.P.'s layout (as pictured in Jeff's photographs) in the Assembly Manual is on page #66 dated 11/7/66
**** denotes an unknown change during the production year