While opinions are good and conventional widsom should be challenged, I think you are barking up the wrong tree on the master cylinder. The judges and the MCA has amassed plenty of documentation that confirms that the master cylinders for 1967 and later are of the style that Jeff states. The bolt-on lid is an aftermarket. Jeff, Tim, Charles, and myself have seem 100s and 100s of original Mustangs and we hold firm that the the lid is held on by the wire retaining clip.
What other areas of the MCA rules do you have differing opinions?
Regards,
Pete Morgan