Note too the red paint daubs on the check valve and the booster body as shown in Jeff's images. those were present after a specified date when the check valve was changed over to a different version in the assembly build at San Jose or changed as a recall on earlier cars.
+1. Not a Shelby thing but a Mustang thing
Three variations
1- Pre-recall = no red daubs
2- Red daub on check valve only = built during or after first recall
3- Two daubs (check valve & booster) = built during or after second recall and fix
Of course you will find earlier cars with the later "fixed" check valve and or booster if they returned to the dealer to get the updates/fixes. Just would not be their original parts
There are likely examples that were sitting at Shelby or enroute or at dealerships that may have also recieved the latest upgrade. One of the things to look for those cars would be the day of the first sale. Fix could have been done at the selling dealership predelivery