Gary,
The third box from the left on the second line of 65 and 66 build sheets carried the build codes for the Radio Noise Suppressor, Transistorized Regulator, and Alternator. As Jeff has pointed out previously, these build sheets served all cars that Ford built, not just the Mustangs. It's possible that Ford had a different build process when it came to Mustang noise suppressors. Still, it's strange that the build sheet would specifically mention them, yet they didn't use the code (which for the 66 models appears to be a code 4).
I suppose that a line worker might go by the physical evidence (the presence of a radio). But then I don't know the build sequence of when the under-hood electrical components were placed in relationship to the insertion of the radio - or as Jeff points out the radio support brackets and antenna prep.
I'm merely pointing out what I've observed in my build sheet study so that we can advance our knowledge base. If there were two noise suppressors (and I'm not sure of the reason for the redundancy, Ford was certainly cost-conscious enough not to waste their engineering dollars) then there must have been another mechanism and reason for its placement.
1966 build sheets had codes of 5, 6, and 7 respectively for the noise suppressor combined with a 55, 53, or 60 AMP alternator. My assumption is that these alternators weren't equipped with an integral noise suppressor. However, 38, 42, and 45 AMP alternators had no code provisions. Did these units have the built-in suppressors? I believe that only the 38 and 42 AMP alternators were used on Mustangs. And I've only seen codes for the 42 AMP alternator (code 8) on 1966 HiPo cars - one from Metuchen and one from Dearborn.
Pete