Notched?

I don't have any info regarding the purpose of the 'notch' itself, but what the notch indicates is that the bracket in your link is the steeper 35 degree angled bracket used with 270 coil's to clear the air cleaners on '69 Z/28's w/cowl induction, and '70-'72 Z/28's with dual snorkel air cleaners.

If you're using original style air cleaners for the above mentioned cars, the notched bracket is what you need for a stock application.

The pics show a lesser angled bracket with a 25 degree angle for comparison, and brackets with other degrees of angle were also used for different air cleaner applications. Info and pics from former member M21RSZ28.