Wondering what the legality of possession of such a device, without the firearm, would be.
Under federal law a shoelace may be considered an automatic weapon.
The law gets mighty murky when absurd bright line rules are accepted and the enforcement arms of the government are trusted for their discretion. But current law is anything that may be used to convert a semi automatic weapon into an automatic weapon counts as possession of an automatic weapon; even without the automatic weapon it purports to convert, without needing to actually be functional, and existing in whole or part. The specific court ruling was indeed, a shoelace counts. DOJ's position was copy pasted into the court ruling.
State law; depends on the state.