Upper receiver question

[URGILOVERR]

What is this cut out for above the gas tube hole ?

 

[NCgreentick]

Probably an anti-rotation slot for their handguard.

 

[ironman56]

AR Stoner has something similar on some of their uppers that mates with a hole on their handgaurds for a alignment pin, a good idea in my opinion.

 

[alamo5000]

If you Google "wedge lock" there are hand guards that use a pin to prevent rotation.

If I was a betting man I would say the pin goes in a corresponding hole on the hand guard and the other end goes in that notch.