I got my first AR15 parts kit.  I'm not at all familiar with the Stoner design;  all my EBR's are FAL type weapons and I'm in another world now.  My question is this:  I have found two proof marks on the rifle:  the upper has a mark that looks like a keyhole.  The A3 carry handle has a proof mark that looks like the state of maine. (bushmaster?)  I can't find any other proof marks...but I have no idea how to take this this thing apart to get a good look at the barrel inside the handguards.

Quoted: I got my first AR15 parts kit.  I'm not at all familiar with the Stoner design;  all my EBR's are FAL type weapons and I'm in another world now.  My question is this:  I have found two proof marks on the rifle:  the upper has a mark that looks like a keyhole.  The A3 carry handle has a proof mark that looks like the state of maine. (bushmaster?)  I can't find any other proof marks...but I have no idea how to take this this thing apart to get a good look at the barrel inside the handguards.

Short answer is proof marks show who made the forging, not who milled the part.  You will never know unless you bought it directly from the manufacturer.

And they're not proof marks. They're simply ID marks from the companies who turned out the raw forgings----and most everyone uses forgings from the same few forges.