"Upper cowl blackout?"
In the wiper well area? OR the upper part of the firewall?

My 09D of 69 Norwood has two, one where the speedo cable exits the interior and one down behind the floor to cowl seam.

I've only ever seen one car with (3) clips before and it was on a 68 Camaro.

However, the other day I was under my L35 68 Camaro and noticed that it also had three clips.

The norm is two clips, but some bring three.

Yep That’s what I always notice

The ‘69 AIM shows 3 locations, but many cars seem to have only ever had 2 from the factory.