Guys,
Sorry it took me so long to reply and post your pictures.
Anyway, thanks to Scott and Chuck, we have some examples of June '64 production cars.
Here is Chuck's picture. This is a June '64 production car. As you can see, it has the "typical" dual exhaust brake hose bracket <i>and</i> the regular bracket typically found on single exhaust cars. It should also be noted that the dual exhaust bracket is stick welded rather than the typically seen spot welding.
[Blocked Image: https://lh3.googleusercontent.com/-zwB4n-sCi1g/TewKEG3WGvI/AAAAAAAAXnA/YHp5FOf6PxQ/s640/64.5%252520mustang%252520001.jpg]
Scott's June '64 car also has the typical dual exhaust brake hose bracket but <i>not</i> the other bracket. His bracket looks like it is at least partially held on by a screw/not sure how or if it is welded.
[Blocked Image: https://lh5.googleusercontent.com/-hSo4jhTmHx0/TewKMzyNg1I/AAAAAAAAXnY/0FKBDMZWsLI/s640/June%2525201964%252520car.JPG]
Of course, as stated in my original post, my Aug. '64 Dearborn convertible has just the single exhaust brake hose bracket and not the other bracket. Fred also mentions the '66 Dearborn car with both brackets, so obviously this may have been a "hit or miss" type of thing.
Thanks to everyone for their help (and digging up this old post!)