Just pulled the stock exhaust manifolds off and saw these slits in the block. You can see the corresponding carbon buildup on the gasket. Looks like a restrictor of some sort or is that line a evidence of a leak?
I had the right-side header off on my 08 CS to remove broken studs on the flange end and found the same pattern. I wondered at the time if the slit in the head was for thermal reasons. I cleaned up the metal gasket and reused. This was about a year ago and 10K miles, haven't noticed any problems.
Great comparison photo. I noticed your manifold has "bleed through". Not sure what effect that has on anything - just an observation. Looks like mine is the same way. This photo is the manifold-side of the gasket.
If you search back far enough I asked the same question some years ago and never got a definitive answer.
Pretty sure it's the passage the air injection pump feeds air through to the exhaust to help cat light off. The pump is by the AOS on the RH side of the engine. Only on for a short time after start. Guess the amount of gas crossover is small enough to not effect any exhaust tuning.
The slits are a path/recess for the secondary air injection system, to help cat converter light-up @ start.
You've already taken the gaskets off your car -- to install new ones - or reinstall the old - match the gasket carbon track with the path on the headers.
IIRC (big fine-print) the notched corner of the gasket is on the inside - toward the centerline of the car. The driver's side (left) gasket has the notch toward the front of the car, the right, towards the rear.
Again! If I remember correctly ...! Use the carbon track as your guide. The springy tabs on the gasket point up ...