Know a number of original owner car that do not have them but none of them can be sure if the shock tower caps/ front upper shock mounts have not been replaced at some point.
Remember this is Ford thing rather than a Shelby thing since many/most export Mustangs received the same washers welded to the mounts.
Plenty of problems verifying a bolt on part as original after over 40 years of use
Its an interesting detail to collect (if a pattern might arise tracking the Ford VIN's) but agree with Bob that its easiest at this point to have them in place.