Notch in the large aperature....

[Straight Shooter]

What is the notch at the top of the large sight aperature for?
Ive wondered this for years.
Thanks for any info.

[MarkG]

It corresponds to a windage scale on the back of the rear sight base.

[Straight Shooter]

Yes, I realize it is for the windage scale, but many many years ago, I seem to remember it also had another use, and for the life of me I cant recall what it was. Seemed maybe something attached to the sight itself for some reason?
Does this ring a bell to anyone, or maybe Im just mis-remembering?

[urbanpioneer]

It corresponds to the windage scale on the back of the rear sight base.

[RGoose]

I believe it can also be used to correct for the sight's height over bore when shooting a distances less than 10 (15?) yards in very hasty situations.

[swa0210]

Like mk18 said above, it is basically just an index mark for the windage scale.

Even in the marksmanship manual for the M16 platform, it only refers to it as an index mark.