I'm with trotterlg on this one. It looks as if it could be a master control hole for a jig to make the machining tolerances tight enough for drop-in barrels, maybe. I know I am dreaming, but wouldn"t it be nice?
I also thought it might be some safety feature that when the pin is knocked out and a locking pin is inserted, could be used to either lock the barrel down or prevent it from closing, thus rendering it inoperable.
Did you guys get the standard safety device that renders the rifle unusable? Just wondering?
It does just look like another very big wart on the side of the receiver to go along with the other ones. I am sure someone will call CS Monday and find out what the heck it is.