The 66 page (T6-8320) is dated "7-6-65 Supersedes 6-16-65", which is before 66 production started. The change appears to be in reference to a piece of tape (Item 4, ESB-M3G32-A) that was removed. The coding for the change states "Effective With CZU1-PF-500589". I can see the decade "C", and Mustang "Z", and I assume the "U1" indicated Unit 1. The rest may be the number of the Engineering Change.
From the 65 Interior manual, the tape, 1.5 x 1.5, 4 pieces, covered the front seat belt holes when no belts were installed. The odd thing is tape is applied to the back of the carpet.
Jim