I thought folks would be interested to know that 67 Mustangs DO have part numbers and dated coded front bumper brackets.
While this is not important today for Concours, it is for Thouroughbred classes and to help document your car as having original parts.
It may well help in stating your case that the bumper and brackets belong to the car in question. It may also help in stating your car has no collision issues if all the parts up front are correctly dated. This isn't full-proof as a PO might have stumbled upon parts close enough. But every piece of documentation helps.
My left bracket says (upside down):
C7Z B
1 19 ꓷ2
My right bracket says (upside down)
C 7Z B
(month gone due to Shelby-specific frame bolt hole taking out the number) 3 D 2
This pre-dates my 3/2/67 car date nicely.