I think your assumption is correct, but the chances of bolt head and the lug abutments being out of square is very,very slim. The front end of the receiver is machined in one fixturing, so the lug abutments, the receiver threads and the face of the receiver is all square to one another, at least on actions made on the new machinery.
It would be very difficult to deliberately make them out of square. The bolt heads are the most consistent components for holding tolerance, but most will suffer from a slight dish in the bolt face from the vibratory polishing operation. Typically this dish is less than .002", and doesn't hurt accuracy, but sometimes it will contribute to primer cratering.
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