Though it had been discussed in earlier threads the subject came up in a recent thread where members were trying help a new member determine if his car was originally n automatic or manual transmission car.
The slot is a vertical hole through the firewall directly below the master cylinder where on manual transmission cars a bracket was inserted from the interior side. On earlier cars no slot was present and there was only a spring on the lower end of the clutch equalizer arm. With the change, on manual cars, a new equalizer design with a hole for the upper spring and the new spring (painted black) was installed also. On automatic equipped cars a simple rubber oval plug was installed from the engine compartment side except on a handful of cars built right after the introduction of the new firewall design
Early firewalls without slot for bracket
++++++++++++++++AFTER CHANGE +++++++++++++++++++++++++++++++++One of the lowest VIN's with the slot (lower than others without the slot) with black friction tape over the hole
Should be mentioned that the early versions of the MPC note that the change took place on 2/14/66 and the survey does match the general time period though we can't be certain that all earlier firewalls were removed from the plant and the inventory was not used up past that date.
Slightly later example with the typically found oval plug when the hole was not needed for the transmission ordered (automatic)
Example of the later version with the slot and with the bracket installed and in use
Jim posted pictures of the bracket as well as measurements in the thread below
http://www.concoursmustang.com/forum/index.php?topic=23641.0
