Thumb relief cut purpose?

[smoothy8500]

I understand the clip-slot purpose, but what function does the thumb relief cut on the M40 A series do?

 

[Macca]

The thumb scallop,like the name implies,means you can push down the rounds closer to the clip & not bang your thumb on the rear of the ejection port.
What does the thumb scallop & clip slot on the M40 series do?
Nothing.