AR-15 Upper question

[dfariswheel]

I was looking at my buddies new Rock River Match rifle, and a couple of AR-15 books he brought.
I noticed something I've wondered about before.

On nearly all AR-15 and AR-10 type rifles there's a raised vertical mark in the upper receiver, over the locking pin bulge.

Any idea what this is for. It must serve a purpose since it's on almost all of them.

 

[Bazooka Joe71]

Ha, I've never noticed that before...You're too curious.

I don't see what purpose it could possibly serve.

 

[Walkalong]

My RRA has it too. Interesting. It must be there for a reason.

 

[rcmodel]

It's always been there, since the first Colts.

Perhaps Gene Stoner laid his drafting pin down on a blueprint and left a smudge?

Or maybe the Colt forging die had a scratch there, so they cleaned it up a little with a die sinkers file?

It could be a locating point to set up the forging in the milling machine jig?

Or more likely, it could be to reinforce the thin web section against getting dented in quite as easily.

rcmodel