What is the purpose of the countersunk area on the front lug on an M16A1 upper?

Thanks

I asked that same question once

Early slab side lowers had no detent to retain the front pivot pin, designers installed a ball detent on the pivot pin itself and relieved the receiver boss to accept the detent thus holding the pin in place. Because they only releieved one side of the upper one had to know on which side the relief was located to avoid putting the pin in backwards. During the refinement of te AR10 design it was believed that removing the upper form the lower would not be a regular occurance. The addition of the partial fence on early  M16s was purely to accomodate the front pivot detent spring and plunger.

HTH
AR10-Dan in Ga