I don't know of a single factory AR lower receiver with a slick sided magazine well like yours, without any ridge running around the bottom edge for strength and to allow for a beveled surface for quicker mag insertions.
Even the original ArmaLite rifles from the 1950s and Colt's SP1 from the 1960s had such a ridge at the bottom of the magazine opening.
That was a challenging one,
and what's really cool is that this
"tough nut to crack" is part of your own personal collection,
and made right here in the metro Atlanta area.