I have examples in my wrecking yard of both San Jose and Dearborn Cougars with miss-matched brackets, one of each. I would expect that the Mustangs also shared this trait? I don't have enough examples to draw a conclusion, but it looks like May for the Dearborn cars, and June for San Jose. This same bracket with the curved front edge was used late 1968-70. See examples of mixed brackets.