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Is there a simple way or measurement to tell the differance between 68-74 and 75 up headlight housing support brackets without fitting them up to a car?
If you mean the actual support, It changed in the 68-74 years as well. It will function, but you need to use spacer blocks between the front mount of the support and the nose attaching points.
The blocks usually come with the support. The part number is the same even though the piece differs. I suspect the change came about in 73 with the advent of the rubber bumper.
I can't really say what the difference is in the 75 and later piece.