I noticed that on one of the two Brembo discs I purchased, there is an arch-shaped notch about 2 mm deep cut into the outer edge. Is this normal or is it a defect?
If you see a notch about 2 mm deep on the Brembo disc you bought, there's nothing to worry about as this is not a defect.
This notch is simply where a small quantity of material has been machined from the disc during production to ensure that the disc itself is perfectly balanced - as any imbalance can produce undesirable vibration.
All discs are checked and, if necessary, balanced automatically during production.
Discs are checked on an electronic test bench which spins the disc and measures any imbalance. If any imbalance is detected, the excess material is ground away to balance the disc perfectly.
Not all discs need correction, and the majority are already perfectly balanced, making it unnecessary to machine material from the outer edge. This means that some discs may have a notch while others do not.