why did they cut the threads

[ActionYobbo]

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.

by your description I assume these are not Lithgow produced bodies you have. I have several lithgows and they dont have any groove in the threads on the extractor side but the imbel's and FN's do (must be a metric thing).

I wondered if it was a safety feature like a built in fail point because there is really no other practical reason for it to be there. That part of the bolt recess can be completed from the rear just like the extractor recess was done. To me it makes no sense to do the extractor side from the rear and then just finish off that corner from the front and run into the threads while doing it.