What I'd suspect is happening: In California you need the state DOJ's permission to mill an 80% lower, then they give you a serial number that you have to engr

What I'd suspect is happening: In California you need the state DOJ's permission to mill an 80% lower, then they give you a serial number that you have to engrave on it within 10 days. Other states have different laws. Some are similar to CA but most are less restrictive. There are places where it is true to say that "Such firearms have no serial numbers, and ... legally bypass background checks and registration regulations." But the author is conflating the national laws with the laws for manufacturing in California (without mentioning that what happened in California is actually multiple felonies). It's either ignorant, unintentionally misleading, or pushing a narrative.