Don't believe that the application of the Thermactor was changed just the design was altered - Didn't want anyone to get the idea that the Thermactor was discontinued mid year 
Page 8-43 of Mannel's Mustang and Ford Small Block V-8 is the following, "To further assist in emissions reductions on M/T equipped engines, Ford/ Mercury also incorporated a T/E system, regardless of whether the cars were built in, or intended for sale in, the State of California. This policy continued until late into the production year, at which time the T/E system was phased out."
The T/E system was not used on late production M/T Mustangs.

Danny