I have several other 67's that don't have these plates and came factory with dual exhaust which is why I thought it might be a specific plant issue or running production change.
These reinforcements would have been factory and original for any 67 with factory dual exhaust
Not a running change in 67 - were installed in the years prior and the years following
As to your comment that you having cars with factory duals and no braces I guess you have Marti reports for those vehicles and there has been no repairs or other things in the cars past (read all factory sheet metal) Not unusual (especially in years past) for repair personnel to skip, miss or forget to add these during repairs.
Guess you've checked the panel dates and rear frame rails to confirm other clues. Might be good start for another thread,