why did they cut the threads

[Woodsy]

I have just examined 6 FAL bodies I have lying around. The groove aligns perfectly with the upper left corner (looking forward) of the squared bolt recess in the receiver ring, so it has clearly been formed as part of the broaching process to form the bolt recess. There is also another similar groove on the opposite side. The extractor recess has clearly been cut from the rear of the action with a special end mill as it comes to a square blind end before the barrel thread, and the axis of this cut is angled upwards towards the rear by about 10 degrees. This extractor cut could only be made by a special fixture and custom tooling in a milling machine.