When used receivers were received by the Springfield NM instruction team, they were separated into SA and non-SA. The SAs were set aside for building. The students were told to select from the non-SA pile to practice on and build a rifle. I'm guessing a good student rifle or two might have been taken into the pipeline. There was also a period when the SA NM team was desperate for receivers, I think they relaxed their SA-only prejudice for a few guns.
Attachment 96375