Haven't been able to place a specific point of the change over or the introduction of the newer style. The fact that there were plenty of the earlier versions on shelves of dealerships, regular maintenance schedules, it being a fairly often changed out part and the possibility that they were still being used at the plant makes for a challenge IMHO
Right now, for me it appears to be somewhere in the Dec 65 - Jan 66 built cars. Need to check through all the Mustang data and pictures to see if that can add anything to the discussion