If Mustang is the same as a Cougar year for year, the difference has to do with the link rod from the inside door handle, to the door latch. The link rod, is the long rod with cloth wrapping.
1967 has a square hole with a 'finger' bend down inside, to support the link rod. Ford part number for the link rod C7ZZ-6521940-A.
1968 has a recessed hole, and the link rod has a loop attached and connects to the door. Ford part number for the link rod C8ZZ-6521940-A.
There may be other differences in the stamping too.