https://everygunpart.com/handgun-kits/semi-auto.html

Purely out of curiosity, what is the source of these kits? Someone takes a gun and cuts it up. This makes a lot of sense for guns that cannot be imported as guns. The ugly side of living under the oppressive regime, I get that. But where do these come from? Are they all stolen and/or hot guns, which are cut up in order to dump the S/N?