Ok, first off I am new to the AR game, so plz forgive my ignorance and uneducated terminology in describing this.  I figured I had to start sometime/somewhere in learning eveything.

This weekend I bought an AR at a public auction with a Armalite Eagle Arms 5.56 lower and a Colt 7.62X39 upper.    I have downloaded the user manual off the Internet and proceeded to take it apart and clean it about three times now, making note of what things are called, how they work, etc.  Everything is pretty straightforward except for one thing.    In the lower, there is a red piece of rubber that sits right behind the trigger.  It pops right out and you put it right back in and it kind of lodges in between the sides of the lower.  To help describe it's location, it would be right in front of the hold where the takedown pin is.    It's not in user manual, and I can find no pictures of it on the Internet.

Does this sound familiar to anyone?  Is it legal?  

Any advice would be greatly appreciated.  And if anyone has figured out a workaround on the whole magazine issue for 7.62 I would love to hear it as I am still trying to decided what to do.

I think you are describing an "accu-wedge" it is supposed to keep bad fitting upper / lowers from rattling. I'm not sure if it helps acccuracy other than to take your mind off  the noise and movement of a bad fitting upper / lower while you are trying to make a shot.

Yep, that's it.  Kind of in the shape of a highback chair.  That's where mine is located, but it's just a little farther up than the one in the picture you show.  Thank you!

sits under the takedown pin and spreads the upper and lower apart when closed to take the "play" out of the two.